what is dark matter?

Question

and how do we know it exists?

Answer 1

As I understand it, it's really just a model; the universe appears to be expanding, and the rate of expansion (gained from seeing the apparent change in light frequency reflected from receding planets and stars etc) does not match up with the amount of mass we can "see" - a figure presumably arrived at by knowing the approximate amount of stuff in a given space in the sky, then extrapolating that to include the rest of the universe. There isn't enough matter that we can see to account for the universe expansion rate, therefore there must be stuff we can't sense with current equipment i.e. dark matter.

Put simply, dark matter may be matter that emits no electromagnetic radiation...... or it may be that the current description of the universe is very slightly flawed and there is no dark matter at all!

Answer 2

Physicists have done all their calculations and worked out how much matter they know there is in the universe, and also how much matter there should be for everything to make sense (gravity, etc). Unfortunately the two don't match. In fact they are out by a massive factor. So instead of saying their theories are flawed, they have invented 'dark matter' - stuff throughout the universe which has mass (weight) but doesn't actually seem to exist. Sleight of hand or what?

Answer 3

Dark matter is matter that doesn't radiate energy, but that's not what you're looking for.

What you're looking for is "dark matter" as an abbreviation for the term "exotic dark matter." Galaxies appear to hold together better than gravity should be holding them together. "Exotic dark matter" is a fudge factor for explaining this odd phenomenon, and we can't observe it; it's invisible, and the only interaction it has with the universe is that it has gravity.
We don't really know that it exists because we can't observe it directly. Recent observations on certain galaxies, though, agree with the hypothesis that it does exist.

Answer 4

dark matter in the universe is matter that we can not see (if we could see it, it would not be called dark). one of the ways that we know that it exists is by its effect on other objects that we can see. for example, consider a star orbiting a black hole. the star is visible while the black hole is dark.

Answer 5

I don't know what it is, but scientists know something is there because of the effect of gravity on galaxies cannot be explained by normal matter alone.

Answer 6

dark matter is science for "we don't know"
there are certain things science cant explain
so scientists and physicists come up with wacky
theories like quantum mechanics and evolution

Answer 7

Because Nibbler leaves little marble-sized Dark Matter turds that weigh a ton in his litter box! DUH!

Answer 8

there is no actual way to prove it right. we just assume.