What do scientists believe happen in a black hole?

Collapse of matter.

Once anything, including the other half of Hawking radiation crosses the event horizon it stays within the event horizon. Theoretical physicists think everything collapses into a singularity as this is a mathematical solution, but there could be a large sphere made up of subatomic particles. Nobody knows for sure. Inside the event horizon should be extremely hot as heat cannot escape past the event horizon and more heat, which is lower frequency electromagnetic radiation is continually "going into" the black hole

First thing you need to learn about a Black Hole is that it is NOT a hole.

So there is no IN.

A Black Hole is nothing more than a DEAD STAR with a huge Mass where the Gravity is so strong light can not escape.

Basically, everything in the radius of the really strong gravity of the star gets crunched really small but the gravity stays the same. As a result places where there would normally be matter suck in even light. At the centre however is just a pinprick of matter so dense and heavy that you could put it on earth and it would make a hole straight through

Things get crunched into small areas of space and most energy does not escape.

Bad things.