For the last semester I studied general relativity and quantum field theory. You may recognize these as words Rick & Morty fans throw around to seem clever, but here's the general gist: QFT is the most accurate physical theory of the fundamental forces in nature and the particles that act on them, while general relativity describes how spacetime warps in the presence of large amounts of energy, mass for example.
Why am I telling you this? Because as it turns out, the two deeply hate each other. You try to apply the results of GR to QFT? Well, good luck calculating anything, because it's infinity now. Want to make a particle out of gravitational waves, as we do with all fundamental waves? Well good luck with that too, because the calculations there are also infinity.
Now, infinities, those aren't something we just give up from, not in QFT. There's something called "renormalization", which is a well-developed mathematical approach to making the infinities go away. Sounds good, right? Well, gravitons can't be renormalized. This is mathematically proven.
Enter string theorists, who seem to have a well-developed model of quantum gravity. What's the catch? Well the catch is we need to have about 8 extra dimensions. Could be 21. They are, however, pretty small dimensions. Also, it's incredibly general. How general? There are about 10^500 possible versions of the vacuum state in string theory, that is 10000... with 500 zeroes. How do we know which one is right? We don't, that's how. How important is it to know? It's essential, of course.
Then there's experimental predictions. When you're doing science, you're supposed to check your theory with an experiment that can be done in real life, so you can make a prediction of what'll happen and see how you're wrong. What does string theory predict? Supersymmetry. Has it shown up yet? No.
At this point I've rambled pretty far off topic so back to the original point: Gravity sucks.