
Will We Ever Know What Dark Matter Is? - dnetesn
http://cosmos.nautil.us/feature/133/will-we-ever-know-what-dark-matter-is
======
Aardwolf
I don't think we will ever know the answers to fundamental philosophical
questions such as why or how the universe exists.

However, dark matter is measurable physics. I'm sure if technology and
knowledge keep getting better we'll be able to solve this one, but it won't
help with the aforementioned fundamental philosophical questions.

~~~
pavel_lishin
The how is likely just another question of physics; I recall watching some PBS
special that hypothesized some collision of higher-dimensional branes.

~~~
StyloBill
So how do these branes exist? The fundamental question remains.

~~~
iaw
I don't think the fundamental question is how these branes exist.

The fundamental question is: what's the most basic truths that we can ever
know?

If there are hyperdimensional branes the likelihood is that we wont be able to
learn much about their origins because of their relationship to our universe.
An acceptable answer then becomes "We can't know how they exist..."

------
bronlund
Most people like to think that science is all about sharing information and
that the popular theories are as far as mankind has come to understand the
universe. Sadly this is not true. For every scientific discovery which has the
potential to be used as a weapon, there will be forces trying to keep it
hidden.

Instead of looking forward, it may sometimes be more productive to look
backwards - especially pre-Einstein. James Clerk Maxwell and his luminiferous
aether is as good as theory, if not better, than any modern explanation about
what's out there. He even provided us with the math describing it (though is
was removed later on).

The years after, towards the end of the nineteen century, is full of
scientists claiming incredible discoveries - like anti-gravity, free energy,
wireless transfer of information as well as power, apparatus for remote
viewing and more. Nikola Tesla being the most famous I guess. Think how
different the last hundred years would have been if any of it had reached the
public domain.

On the other hand; making an atom bomb isn't really that complicated (the
difficult part is surviving doing it). If every person you know had one, that
would probably not work out. My point being that there are arguments for
keeping information restricted. Put capital in the mix and it's bound to be
some collusion.

Imagine you had a grid of antennas and that you could adjust the phase and
frequency of each as to get the signals from all of them to be in-phase at a
specific point. Everyone visiting this site could probably think of a handful
of ways to do exactly that. Then imagine you had a lot of antennas and a lot
of juice - you could more or less focus an insane amount of energy wherever
you wanted on this planet without anyone being any wiser. What would you do
with a power like that? Would you share?

~~~
millstone
> James Clerk Maxwell and his luminiferous aether is as good as theory, if not
> better, than any modern explanation about what's out there

No, it's a disproven theory that has been superseded by the far more elegant
notion of Lorentz symmetry.

> The years after, towards the end of the nineteen century, is full of
> scientists claiming incredible discoveries

Cranks are a constant presence.

> Imagine you had a grid of antennas and that you could adjust the phase and
> frequency of each as to get the signals from all of them to be in-phase at a
> specific point.

I look forward to the calculation here!

------
pavel_lishin
My favorite pet theory (head-canon may be a more appropriate term) is that
Dark Matter is a set of matter that's just as rich and interactive as ours -
made up of particles that follow the analogue of our laws, with weak, strong
and electromagnetic forces - but slightly different, so that _that_ parallel
universe is full of intelligent life, investigating "shadow matter" that only
interacts via gravity, wondering how sad and empty and dark it must be for
those particles that make up only 10% of the universe.

~~~
Florin_Andrei
If it interacted with itself in ways other than just gravity, it would make
clumps. The problem is, it doesn't make clumps. Its distribution is very
clearly non-clumpy.

Also, if this was gravity leaking between parallel universes, I believe it
would produce measurable artifacts, and therefore could be tested.

~~~
flukus
Can we say it doesn't clump or can we only say it doesn't clump on the
galactic scale? Could it clump but then go into it's equivalent of a super
nova with very small clumps?

~~~
raattgift
This is a pretty good question.

There is a limit to the changes in the momentum of cold dark matter before it
produces visible effects in galaxies of different ages. Explosive processes in
the dark matter sector must be rare, whatever their mechanism and whatever
their daughter products (even if it quickly decays to cold dark matter again,
but on a different trajectory).

------
platz
I like to look through the lens of quantum field theory instead of
'particles'.

Dark matter may be some field or interaction of existing fields that we don't
understand yet.

------
acqq
Yes, up to now there's nothing directly detected, but we should also not
forget that just two years ago a pessimistic article could have been written
about the detection of the gravitational waves, this is how it could have
looked like: "predicted 100 years ago, never detected, even if there was some
investment in the detectors and specific scientific work for 40 years, and in
spite of some indirect evidence." It is also easy imaginable somebody claiming
that LIGO shouldn't be funded only to "measure the change of a 4 km length in
a tiny fraction of the size of a single proton."

But then... hundred years after the prediction, the magnificent thing did
happen:

[https://en.wikipedia.org/wiki/First_observation_of_gravitati...](https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves)

And then another one:

[https://en.wikipedia.org/wiki/GW151226](https://en.wikipedia.org/wiki/GW151226)

------
Figs
Have we already ruled out the possibility of dark matter just being huge
numbers of black holes without notable accretion disks?

~~~
nol13
Not completely, but seemingly unlikely.

[http://resonaances.blogspot.ch/2016/06/black-hole-dark-
matte...](http://resonaances.blogspot.ch/2016/06/black-hole-dark-matter.html)

~~~
Figs
Thanks for the link. Just found this one from NASA as well:
[https://www.nasa.gov/feature/goddard/2016/nasa-scientist-
sug...](https://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-
possible-link-between-primordial-black-holes-and-dark-matter)

So, it seems like it's still an open possibility, and even better, it might be
resolvable one way or the other in the near future -- at least for the
primordial black hole hypothesis.

------
mattlogan
Having just finished reading Lisa Randall's "Dark Matter and the Dinosaurs," I
was a bit disappointed to not see any mention of her new "dark disk"
hypothesis. Maybe her research is a bit more "fringe," but it seems like all
of these experiments are pretty far out there.

The "dark disk" hypothesis basically says that there are actually multiple
types of dark matter particles, and there's a disk made of heavier or more
strongly interacting dark matter particles in a disk shape overlapping with
the milky way disk.

More on that here: [https://www.quantamagazine.org/20160412-debate-
intensifies-o...](https://www.quantamagazine.org/20160412-debate-intensifies-
over-dark-disk-theory/)

------
twilightfog
Another promising theory is that dark matter isn't even necessary, we only
assume its present due to its gravitation effect, and that can be explained by
other phenomenon.

Gravity decreases in proportion to distance squared between objects, but its
not observed beyond a certain distance. To compensate, that space was assumed
to be filled with dark matter. This theory suggests that gravity switches from
a inverse-square law to a different set of rules after a certain distance.

[https://www.quantamagazine.org/20161129-verlinde-gravity-
dar...](https://www.quantamagazine.org/20161129-verlinde-gravity-dark-matter/)

~~~
lutusp
> Gravity decreases in proportion to distance squared between objects, but its
> not observed beyond a certain distance.

This isn't true. F = GMmr^-2 has infinite range. This is true from a
theoretical standpoint, and it's also borne out in observations. The Andromeda
Galaxy, presently 2.5 million light-years away, is being pulled toward our
galaxy and will eventually collide with it. Observations on an even larger
scale support the idea that the equation has infinite range.

The above theoretical construct could in principle be contradicted by
observations, and if it were, someone would win a Nobel Prize, so there's
every incentive to find persuasive evidence that contradicts it.

~~~
martincmartin
_The Andromeda Galaxy, presently 2.5 million light-years away, is being pulled
toward our galaxy and will eventually collide with it._

To support an inverse square law, we'd need to know the mass of our galaxy,
and the mass of the Andromeda galaxy. We have the speed, computed from various
blueshift methods. We'd also need to know the rate at which the speed changes.

Do we know any of those three? The mass of the Andromeda galaxy is very
different if you assume its filled with dark matter, vs. only has visible
matter. Which mass is used to confirm the inverse square law? If it includes
dark matter, you have circular reasoning: dark matter is hypothesized to make
the inverse square law work within the Andromeda galaxy. If it doesn't include
dark matter, then why does the dark matter in the Andromeda galaxy contribute
to its rotational motion, but not the attraction to the Milky Way?

~~~
lutusp
My reply was only in response to this comment --

> Gravity decreases in proportion to distance squared between objects, _but
> its not observed beyond a certain distance._

\-- not to the question of what detailed properties gravitation possesses.

~~~
martincmartin
Ah. I took the comment to say the inverse square isn't observed beyond a
certain distance, not that gravity wasn't observed at all beyond a certain
distance.

------
digler999
sort of OT of the post, but on topic of all the cool discussions about physics
we've had here, I want to share one my favorite channels on youtube[1]. Eugene
Khutoryansky has put together an amazing collection of very intuitive
explanations of all kinds of physics, including cosmology. I also recommend
his series on math and electromagnetics.

[1]
[https://www.youtube.com/user/EugeneKhutoryansky](https://www.youtube.com/user/EugeneKhutoryansky)

------
Zigurd
Dark matter as particles, rather than some undiscovered gravitational effect
at galactic scale, seems like a difficult case to make. If it is particles,
the distribution of dark matter in a galaxy has to be highly dependent on the
distribution of ordinary matter, or vica versa, and yet there is no
interaction. I'd bet on gravity waves and/or quantum mechanical gravity
effects leading to an explanation, rather than the discovery of a new class of
matter.

~~~
Florin_Andrei
> _If it is particles, the distribution of dark matter in a galaxy has to be
> highly dependent on the distribution of ordinary matter, or vica versa, and
> yet there is no interaction._

Incorrect. The distributions do appear to be interdependent, in exactly the
way you'd expect if the two kinds of "matter" interacted only gravitationally.

~~~
Zigurd
Yes, that's true. But are there arrangements of dark matter that are not
holding visible matter galaxies together? Does dark matter appear anywhere
else?

~~~
Florin_Andrei
How would we know? The only thing it would do is create some extra gravity
where there is no visible source of it.

You could infer its existence from indirect effects, but it would be extremely
tricky.

~~~
Zigurd
Gravity lensing?

~~~
raattgift
Yes, that's how we'd infer the presence of an overdensity of dark matter in
the absence of visible matter : a background being lensed by an invisible
foreground.

There are searches for this which are possible because Einstein rings are well
understood, and those appear in particular alignments such that it would be
hard to miss the foreground source if it were at all visible. Other lensing
structures are subject to tracing error; that is, you lose the ability to
distinguish between effects from alignment and effects from the distribution
of foreground mass and the distance to the background visible objects.

One major problem is that dark matter overdensities need not be in exact
alignments, or produce readily detectable Einstein rings. Moreover, the
overdensities must not be that "over" or ordinary matter would have been drawn
in, and we could see _that_. (Which would be neat as it would offer evidence
on how early galaxies formed in the first place, and narrow out of contention
lots of modified gravity proposals.)

There is a debate over the large scale structure of dark matter, with lots of
ideas provoked by the X-ray clusters Abell 222 & 223 among others. There may
be a "web-like" structure to dark matter where the filaments can be very large
compared to the scales of individual galaxies but still quite narrow compared
to the less dense spaces between them. These structures appear pretty commonly
in simulations grounded in the standard cosmology, although it's not wholly
clear why (they may be an artifact of numerical methods, for example).

[http://www.nature.com/nature/journal/v487/n7406/fig_tab/natu...](http://www.nature.com/nature/journal/v487/n7406/fig_tab/nature11224_F1.html)

There are ground- and space-based observatories in the pipeline that will over
the next few years narrow the possibilities for various kinds of large scale
structure of extra-galactic/extra-galaxy-cluster dark matter.

However, "it's extremely tricky" is fair for now.

------
WhoBeI
Of course we will or rather we will understand what causes the phenomenon
which we attribute to dark matter.

I don't really see the failures mentioned in the article as failures. We had
an idea about how nature works, tested it and showed that we were wrong. It's
just the scientific method at work.

~~~
Florin_Andrei
Yup. This is how the sausage is made (the scientific sausage). Some do not
like it, but others find it fascinating.

------
ww520
Two crazy ideas:

1\. Dark matter is the "gravity echo" from the past. Gravity is the structural
bending of spacetime by mass. The effect of the bending is supposed to travel
at the speed of light. What if the effect has been delayed and is added to the
present effect?

2\. When a mass is moving through spacetime, the bending of spacetime is
asymmetric in the leading and trailing directions, where the leading direction
forms a leading crust and trailing forms a flatter tail, just like the blue-
shift and red-shift effects on light. The leading gravity wave crust adds to
the strength of the gravity.

------
tyfon
Personally I think that "dark matter" is not matter at all, but ripples that
form valleys and hills in the space-time membrane like a plastic bag that has
been stretched (by the big bang) and crumpled slightly back.

But I am not a physicist so it is purely speculation from my part and I have
no math to back it up. It's just the way I picture it in my head :)

------
bborud
The Big Fudge Factor In The Sky Without Which We Look Silly.

------
sunstone
We will if it isn't.

------
arca_vorago
edit: got reemed for even attempting, read below

~~~
russdill
Your theory seems to be based on pop-science understanding of concepts like
entanglement. A physicist listening to you would just smile and nod their
head. There is no need for particles to pass through a single point for
entanglement to be a thing. Both the standard model and quantum theory tell us
that such an event isn't possible. And as far as I understand, you don't have
matter until 10^-12s, well after cosmic inflation.

Quantum entanglement is not a force, does not cause particles to do things,
and cannot be used to build a quantum internet. It merely refers to the fact
that although quantum states cannot be known till measured, even distantly
separated particles will satisfy quantum relations (opposite spin for two
particles, etc). They satisfy them in such a way that they either somehow
communicated, knew all along, or other possible strange options discussed in
multiverse theory. Bell's inequalities prove that they did not know all along.

Dark matter can't be ordinary matter in a second universe because it doesn't
interact with itself. It'd have to be dark matter in another universe. Which
would be indistinguishable from dark matter in this universe.

Nothing in physics seeks to do anything. The idea that matter would get
together and plan to form black holes so that it can transition into some
other state is just silly.

You are trying to solve problems that don't exist, with physics that don't
exist. Please, read up on:

[https://en.wikipedia.org/wiki/Pauli_exclusion_principle](https://en.wikipedia.org/wiki/Pauli_exclusion_principle)
[https://en.wikipedia.org/wiki/Uncertainty_principle](https://en.wikipedia.org/wiki/Uncertainty_principle)
[https://en.wikipedia.org/wiki/Chronology_of_the_universe](https://en.wikipedia.org/wiki/Chronology_of_the_universe)
[https://en.wikipedia.org/wiki/Quantum_entanglement](https://en.wikipedia.org/wiki/Quantum_entanglement)
[https://en.wikipedia.org/wiki/Bell's_theorem](https://en.wikipedia.org/wiki/Bell's_theorem)
[https://en.wikipedia.org/wiki/Dark_matter](https://en.wikipedia.org/wiki/Dark_matter)

~~~
arca_vorago
I've taken the criticism received and edited my comment, but after reading
your comment again I think you are being overly critical.

"There is no need for particles to pass through a single point for
entanglement to be a thing."

I never said there was, merely proposed it as a possibility.

"Quantum entanglement is not a force, does not cause particles to do things,
and cannot be used to build a quantum internet."

Not with classical bits, but theoretically building a quantum internet is
possible if we ever figure out how to transmit data via qubits. (as per
articles linked)

"Dark matter can't be ordinary matter in a second universe because it doesn't
interact with itself."

I don't follow your reasoning here, is there a law that more precisely backs
up this statement?

"You are trying to solve problems that don't exist"

On the contrary, I am just using my imagination to attempt to grasp a possible
version of grand unified theory, which as far as I know is still something we
are trying to work on.

"with physics that don't exist."

How do you know that if you don't know the answer to what dark matter is in
the first place? Science isn't just about what we already know, it's about
creating falsifiable and testable hypothesis outside the current knowledge-set
to expand the currently accepted knowledge set. To criticise hypothesis on
content and methodology is valid, but to criticise it simply because it
doesn't conform to the current knowledge set is short sighted imho.

~~~
Florin_Andrei
> _I am just using my imagination to attempt to grasp a possible version of
> grand unified theory, which as far as I know is still something we are
> trying to work on._

You're attempting to write a super-duper machine learning giant app without
having bothered previously to even learn how to code. Start by learning how to
turn on the computer first, and go from there.

It's easy to kick back and dream about grand theories. It's also easy to
forget the steep road of learning that would allow you to even approach that
realm.

------
ClayFerguson
There is no evidence that dark matter even exists other than the fact that our
universe is not expanding at the rate we think it should, based on our current
theory of gravity and how much mass we can account for based on the radiation
_currently_ reaching us (light). I think it's far more likely that our theory
is incomplete rather than some whole new class of invisible matter/energy
being conjured into existence just to counter-balance our 1) wrong equations
and/or 2) wrong observations. So many non-scientists think dark matter is
proven. It isn't. It's nothing but pure conjecture.

~~~
magicalist
> _There is no evidence that dark matter even exists other than..._

 _Most_ of our theories about the universe are based on evidence from
secondary, tertiary, etc effects. It's often the best we can do.

I don't think you'd find anyone who would object to the idea that "our theory
is incomplete" ("all models are wrong", etc), so it seems like your main
objection seems to be that we've given a name to a family of theories that
attempt to explain the phenomenon we've grouped under "dark matter".

> _I think it 's far more likely that our theory is incomplete rather than
> some whole new class of invisible matter/energy being conjured into
> existence just to counter-balance our 1) wrong equations and/or 2) wrong
> observations_

It's rather human to make an estimate of likelihood based on how long ago
something was conjectured to exist :) Moreover "wrong equations" and/or "wrong
observations" are part of the very theories attempting to explain our current
state of knowledge.

> _So many non-scientists think dark matter is proven. It isn 't. It's nothing
> but pure conjecture._

Actually, usually the issue with these discussions are with scientifically
literate folks who read a Scientific American article on dark matter in 1998
and have engaged with that as a strawman ever since. Somewhat tongue-in-cheek,
but it is odd to me how many people are convinced they're bringing light to
the darkness with these kinds of comments.

------
ifdefdebug
A bit off-topic, but not totally: I am so sick of nautil.us spamming my back
button into oblivion that I won't click a nautil.us link never more, until
they fix that and someone here say it is fixed.

~~~
delecti
I normally just open things in a new tab, avoids that problem entirely.

But just for you, I clicked the link normally, waited for it to load, hit
back, and ended right back up at the HN page. So either an adblocker fixes the
problem, or they have.

~~~
mikeash
They seem to be adding history items when you scroll. Try scrolling down the
page some, then going back. It took me about ten clicks to get back when I
tried it.

~~~
delecti
You seem to be correct.

Though the new tab approach still works.

~~~
mikeash
Yeah, opening in a new tab can prevent all sorts of idiocy.

------
ClayFerguson
So much of what you said is hilariously wrong. Probably the most acute example
is your claim "Standard Model is mute on gravity". Are you kidding me? Page
one of any book of the Standard Model discusses the 4 fundamental forces.
Guess what one of them is: GRAVITY.

~~~
dang
You conducted yourself so nastily in this thread that we've banned this
account. This sort of flamewar is exactly what we're hoping to avoid on this
site.

We detached this subthread from
[https://news.ycombinator.com/item?id=13606710](https://news.ycombinator.com/item?id=13606710)
and marked it off-topic.

------
lutusp
I wish nautil.us would fix their broken page design so it doesn't fill one's
browser history with duplicate links and keep one from leaving the site. This
isn't a design choice, it's parasitic.

------
lngnmn
We know what it is. It is a socially constructed abstract concept, like "god"
or "afterlife" or "pure substances" of Spinoza.

BTW, the "substance propositions" Spinoza has been developed are perfectly
sound, except one small flaw - no such phenomena as pure substance ever
existed. Dark Matter and related crap is of exactly the same nature - much
more sophisticated phantoms of the mind supported by a sect of believers.

~~~
iaw
I've downvoted your comment because your tone isn't constructive and you're
making a rather illogical jump to the philosophical arguments of Spinoza in
comparison to questions around modern measurable/observable phenomena in
science.

If you'd like to make a less hostile post with a clearer justification of your
reasoning I'd be more than happy to debate the subject with you.

~~~
lngnmn
I do not have anything to add. As a student of philosophy I have seen way to
many socially constructed chimeras and memes to not notice modern ones.

The only criteria for establishing a scientific truth is still a replicable
experiment which proves an _implementation_ according to some established law.
Any simulation does not account for such kind of experiment, being mere a
"cartoon" made according to some abstract model. The difference between
experiment and simulation is of the same kind as between a cartoon and actual
reality.

~~~
iaw
> " As a student of philosophy I have seen way to many socially constructed
> chimeras and memes to not notice modern ones."

There's a willful ignorance in this sentence that belies your youth and
intelligence. I suspect you don't have an adequate background to understand
the observational evidence for our expectation of dark matter (I barely do to
be quite honest). I don't think you can fairly dismiss offhand the conclusions
drawn by hundreds of expert physicists based on your experience.

I've known more than one physicist that were also well-studied students of
philosophy. If you have reached a brilliant and well-justified conclusion that
they failed to see you should clearly write it to justify your case.

>"The only criteria for establishing a scientific truth is still a replicable
experiment which proves an implementation according to some established law.
Any simulation does not account for such kind of experiment, being mere a
"cartoon" made according to some abstract model. The difference between
experiment and simulation is of the same kind as between a cartoon and actual
reality."

Science (and philosophy for that matter) are defined from consistency of
observation. I fail to see your point about an experiment measuring a
"cartoon" when that is the reality in which we intrinsically exist and for
which prediction will provide the most value.

~~~
lngnmn
Try this. This is a particularly good one. Science is definitely not a
consensus of observations. It is a systematic effort to unveil what is.

[http://oyc.yale.edu/ecology-and-evolutionary-
biology/eeb-122...](http://oyc.yale.edu/ecology-and-evolutionary-
biology/eeb-122/lecture-23)

~~~
iaw
I'm not asking you to send me a resource that has led you to your current
conclusions.

I'm asking that you formulate an argument that I can read that will make me
believe that your arguments are well-formulated here. You made a very strong
statement and I think that if you could logically justify it in written form
it would be a valuable contribution to the thread.

~~~
lngnmn
I could only try to clarify the same principles a bit more. Non-falsifiable
and non empirically verified hypothesis does not amount for a science and
constitutes a set of beliefs similar to religion among a community of its
followers. As long as there is absolutely no way to reliably test or even
measure, leave alone empirically validate anything, this set of beliefs is
technically is a non-science and should be explicitly named as mere
speculation.

My first comment was the claim that what we deal with is a socially
constructed phenomena, similar to a religious belief or a mass-hysteria
because it is a) cannot be tested and b) constructed by a certain social
group. Technically, I do not see any contradiction in my claim.

