I’m no specialist of the question, but from what I got, ok the standard model doesn’t match what we can observe in astronomic features, unless we suppose some untrackable heavy weight is there though not measurable with our contemporary tools.
So the basic paths are, we admit there is indeed some entity like this, or we need a new model that can also encompass these observations without new object type.
MOND (Modified Newtonian Dynamics) is a minority (not mainstream) theory in physics, that maybe Newton's law of gravity doesn't work the same over galactic distances. After all, we have only tested the law in length scales smaller than our own solar system.
You also have to know, that the observed rotation speeds of galaxies do not fit with estimated amount of visible stars (and their mass) in the galaxy. We basically can not explain why galaxies rotate at the speeds they do.
MOND suggests that the shape of the gravity law changes, over very large distances.
The mainstream explanation is Dark Matter: Galaxies have more mass than we can see, and that makes them rotate differently that what their number and mass of stars would suggest.
Lambda-Cold-Dark-Matter: Lambda is the name of the cosmological constant, cold dark matter is about the simplest possible model for the unseen dark matter: It is cold (no fast moving particles, meaning no particles like photons that move at relativistic speeds), dark (we cannot see it), and it is matter (provides gravity).
This is the traditional, mainstream cosmological model in its simple form.
I’m no specialist of the question, but from what I got, ok the standard model doesn’t match what we can observe in astronomic features, unless we suppose some untrackable heavy weight is there though not measurable with our contemporary tools.
So the basic paths are, we admit there is indeed some entity like this, or we need a new model that can also encompass these observations without new object type.