Why dark matter/energy? 2 points by krsunny 1769 days ago | hide | past | web | 2 comments | favorite Take for example a firecracker, a hand grenade, a star or anything in our universe that explodes. If you were to measure the rate of expansion over time there would be points where the expansion is accelerating, remaining constant or decelerating. At the point in time when the explosion is accelerating, would you call the force driving that acceleration dark energy/matter? No? So why do we invent dark energy/matter to explain the acceleration of our universes expansion?

 You're asking the wrong question. We didn't invent the concept of dark matter to explain the acceleration of our universe. Though if we have theories that have dark matter, we might as well try to make those theories explain the acceleration at the same time.Here is why we need dark matter. When something is in orbit, from the details of their orbit we can tell how much mass they are orbiting around. When we carry out this calculation for galaxies, we get one mass. We also can estimate the amount of mass that each visible star has, then add it up to get an estimate of how much matter there is that we can see. This gives us another number. The two numbers are off by a factor of 10. The difference is matter that we can't see, which we know has to be there. This is dark matter.Our own solar system suggests that the matter in a solar system is almost all in the visible star. But there are plenty of planet-like things that aren't around stars. We can't see those, and don't know how many of them there might be. However our models suggest that those kinds of objects should be distributed like stars. But we have evidence from the movement of stars around galaxies that dark matter is relatively uniformly distributed through a bigger volume than the visible stars.The most popular (but not only possible!) theory right now is that there are lots of Weakly Interacting Massive Particles (WIMPs) that only weakly interact with normal matter. (There is a lot of speculation about the possible nature of these particles.) Computer models with these result in reasonable distributions of dark matter. But this theory is difficult to prove or disprove directly since, by definition, they don't interact with anything we can build to detect them.
 Right after I posted this I realized I only meant to say dark energy, not dark matter. Why is it speculated that the universe will continue to accelerate its expansion?

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