So mass is what warps spacetime, resulting in gravity. But mass doesn't exert a force to do this? It just happens. Got it.
I'm glad you've got it.
To give you another example, consider electricity. What is the relevant force? Well, there's an electric force that acts on anything that has electric charge. Why do some things have electric charge? Nobody knows. That's just the way the universe is. Electrical force is a property caused by charge, just as gravitational force (if you want to call it that) is a property caused by mass. Nobody knows why. It's just how our universe works. The task of science is to model the processes we observe in nature. These things are among the things we observe.
So, you're right. Things "just happen". Things are the way they are. Start getting used to living in this universe of ours. It is the way it is. You're stuck with it, like it or not, understand it or not.
When a body's mass is so great that it results in a black hole, which eventually crushes that body out of existence, what is left to cause the remaining gravity?
The mass that formed the hole.
By the way, nobody can say whether a body's mass is "crushed out of existence" when it falls into a black hole. After all, we can't go inside to look (or, at least, if we
did go inside we couldn't transmit the news to anybody on the outside).
Iow how can a hole in spacetime have mass?
One way to think about it is that, from the outside, a black hole looks, gravitationally speaking, just like an ordinary lump of matter with a certain mass.
In other words, if you could somehow turn our Sun into a black hole of the same mass right now, the Earth's orbit wouldn't be affected. The Earth would keep right on orbiting the black hole, in the same orbit that it now orbits the Sun. There would be some other nasty side-effects, of course, but gravitationally, we here on Earth wouldn't notice any difference.
Wouldn't it rip a hole in spacetime?
There's no evidence that black holes rip holes in spacetime.
"While black holes are mysterious and exotic, they are also a key consequence of how gravity works: When a lot of mass gets compressed into a small enough space, the resulting object rips the very fabric of space and time, becoming what is called a singularity."---
https://science.nasa.gov/universe/10-questions-you-might-have-about-black-holes/
Eww. I don't know who wrote that article, but it's not correct.
A singularity is a kind of mathematical artifact in some equations. There's no evidence for
physical singularities. There's no evidence that there's an actual physical singularity at the centre of a black hole.
This kind of pop-science FAQ stuff is not hard to find, unfortunately. It tends to give the general public misleading ideas about black holes. The people who write this sort of thing really ought to be more careful.
