Question: why do stars die and what is in a black hole?

Duncan McNicholl answered on 14 Nov 2017:

Much as I hate to disagree with Sajid, it’s totally not dark matter. Stars are actually pretty simple from a physics perspective: they’re hot, and have reactions that go on inside them that produce heat, and hot things expand. Unfortunately, they’re also absolutely gigantic, and big things have significant gravitational fields that tend to make them shrink. For most of its life, a star is in balance between these two things, making just enough heat to stop it from shrinking. The funky stuff starts to happen when they run out of the hydrogen in the middle that’s doing the reacting to make heat. As it runs out, the reactions slow down, and now it’s shrinking more than it’s expanding, so it gets smaller. As it gets smaller, it gets hotter, until it can start to do the same reactions it was doing before, but this time with the helium that was produced in the old reactions, then it ends up back in balance, until the helium is used up, and then it shrinks again, and so on and so forth until either it doesn’t get hot enough to do the next stage of reactions but just sort of stops and then slowly cools down (which leads to a white dwarf), or it carries on until the whole thing explodes in an almighty supernova, giving out more light and energy than all of the other stars in the galaxy for a couple of days or so. If the supernova thing happens, all that is left at the middle is the heaviest elements, and sometimes they’re so dense and have such a lot of gravity pulling them in that they carry on shrinking until they’re basically just a dot with all those elements inside it. If that happens, the gravitational field gets strong enough that even light can’t escape from it, and that’s what we call a black hole. So what’s in a black hole is the remains of a dying star crushed up so tight that it stops light from being able to get out.
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Dark matter, on the other hand, is all the stuff in the universe that we just can’t see: there are some bits of maths that you can do to work out how much mass a galaxy has, and if you do that, and then separately add up all of the mass of the stars and the planets and the black holes and the supernovas, most of the time the two numbers are different by about twenty times. As in, there’s twenty times more stuff than we can see. We don’t really have any idea what it is, but a bunch of cosmologists have come up with a bunch of things it might be, and there are scientists all over the world doing experiments to see if any of them is right.