Question: what is dark matter

Simone Sturniolo answered on 20 Jun 2013:

We don’t really know what dark matter is: that’s why we call it “dark”! Dark matter is a mysterious “something” that astrophysicists have calculated that has to exist in the universe but that we can’t see. Basically, every motion in the universe is regulated by gravity. For example, the Moon goes around the Earth because it’s attracted by Earth’s gravity; similarly, the Earth goes around the Sun, and the Sun moves in circles around the centre of our galaxy (the Milky Way), and so on.
The problem is, when observing galaxies, not all motions make perfectly sense, according to what we know of gravity: some things move as if they were attracted by something that simply… isn’t there. So the idea is that what’s there is dark matter: something that attracts stars and planets, but is invisible to us.
This is not so weird, we see stars because they are very shiny after all. Planets and black holes are already harder to detect, though we can still recognize them from the effects they have on their surroundings. Dark matter could as well be some kind of matter that simply does not interact with light (in other words, it’s perfectly transparent) or other forms of radiation, and that thus we can not see with our telescopes. Hence its mystery. Many scientists are trying to produce particles (for example in LHC) that are as “transparent” as dark matter needs to be, in order to see if it is even possible. Some think that dark matter does not even exist, and that the problem is that our laws of gravity are actually wrong and should be modified. They are a minority, but this could be a possibility too! Anyway, the mystery goes on. The day someone will be able to give a true, final answer to your question he’ll probably win a Nobel prize!