[chousuke]

I'm not an astrophysicist but I think I can provide a reasonably accurate answer anyway.

Basically, you get a black hole when you push matter together tight enough. This happens when some stars die, and the processes inside the star can't counteract its own gravity.

Light is affected by gravity. A black hole is an object whose gravitational "pull" is so powerful that inside a certain radius, everything gets inevitably pulled into it. This causes the event horizon, where even light can't get away.

What's inside the event horizon is not known, as far as I know, except that it has mass, charge and angular momentum

Black holes colliding is essentially no different from any other two objects colliding in space, except for the cataclysmic scale. They behave pretty much like any other object of their mass would, which means you can have two black holes orbiting each other in a binary system just like two stars would.

[samstave]

I really loved this ELI5, thank you.

[chousuke]

I guess it was more of an ELI5 for kids with exceptional vocabulary. :P

One thing I neglected to mention is that the effect of gravity gets weaker with distance, so conversely it must get stronger the closer matter is pressed together. And as it gets stronger, the object gets denser because (in the absence of counteracting forces) its matter is further pulled together, and it becomes a loop until you get a black hole. Beyond that things get weird and we don't know what exactly is going on, but we do know that it happens.

[samstave]

So what the heck is gravity???

And assuming youre pulling mayter together so strongly, what happens to the spatial size of the atoms being pulled into that black hole? Does the physical size of the atoms change? Do they transmogrify into some other substance? Are black holes hot? Or cold?

[chousuke]

As far as I understand it, you can think of spacetime as having a shape. All objects travelling through spacetime must conform to its shape. Einstein's insight was that objects with mass affect this shape of spacetime, and thus there is an apparent force because spacetime itself bends and thus any path through spacetime in that region also changes. It's very weird, but also kind of neat. The effect is very weak though which is why it's only really noticeable around very massive objects.

If it helps, draw a line on paper and bend the paper in various ways and observe how the line changes. You should also be able to find demonstrations on YouTube using a stretched canvas.

As for the size of matter, atoms aren't actually the smallest thing we know of, so there's (relatively speaking) a lot of empty space even inside an atom that you can squeeze out.

As for what happens inside black holes under mind-boggling pressures, while there's reason to suspect that there is such a thing as "the smallest possible space" (a quantum of space) that would act as a limit, but it's not been possible to confirm yet, so it's only speculative, and as such the only intellectually honest answer is "I don't know".

As for the temperature of a black hole, I don't know. I think Hawking radiation implies they have one, but you'll need to find out yourself.