I don’t get that at all, how can a universe just start and finish. It had to be started from something and when it finishes thing just cant disappear. There will always be matter. For example if the universe explodes, what about the debris? There will certainly be elements left behind, dust, bits of rock etc?
You're being misled by the explosion analogy (which itself was originally coined to mock the theory!). There won't be debris left behind if the universe collapses again, since there won't be a universe for the debris to exist in.
I think you're imagining the big bang like you see it on Horizon - a giant explosion in the middle of a big dark space. That's a great analogy, but it's not strictly accurate. Another analogy (again not to be taken too literally) is the idea of the universe being like an expanding balloon. Let's pretend for a moment that we're flat, 2D creatures. Imagine a balloon, and imagine that out 2D flat selves live on the surface of that balloon. That surface of the balloon is the universe, and there is nothing outside of it to us. We can move around the balloon, like we can move around the globe, but we can't move up or down, the words 'up' and 'down' have no meaning for us. We live in a flatland - with me so far?
Now let's imagine the big bang in our flatland. The big bang here corresponds to the inflation of the balloon - imagine this universe starting as a tiny, tiny microscopic speck, then slowly inflating bigger and bigger until we have a full-fledged balloon. At the beginning, the balloon is so tiny that it has no surface area. Our universe is the surface of that balloon, so our universe doesn't exist in a meaningful sense. As the balloon inflates, its surface area gets larger and larger until there is enough space for 2D stars, planets and flat creatures like me and you to form. At the end of this universe*, the balloon deflates again, it's surface are shrinking back down to nothing. There can be nothing left over, because we live on the surface of the balloon, and the surface area shrinks and shrinks until there's nothing left. There's literally nowhere for any debris to be - no spatial dimensions, no time, nothing. The universe is shrunk to a tiny, tiny point where there's no backwards or forwards, no side to side, no up or down, and no forward or backwards in time.
You may need to read that a few times, I've never tried to explain that properly before and my exposition isn't particularly polished yet.
*: To pre-empt any criticism, let me say that I'm assuming a big crunch ending, ignoring accelerating expansion and dark energy, ignoring string theory or entropic fluctuation theories etc etc. That stuff is interesting, but superfluous to the point I'm trying to make here.
Standard model still doesn't explain mavity.
It's not supposed to, it's a particle physics theory.
That said, I take your point. Quantum mavity is still not well-formulated (except in string theory, which itself is incomplete), but we understand classical mavity on large scales pretty well. General Relativity is a bit more established than you give it credit for - it's a stunningly elegant bit of work that describes mavity on large (i.e. non-quantum) scales near perfectly.
