So now we understand that quantum mechanics essentially blasted open how we think about the universe (when it comes to the tiniest of scales). Particles can be waves, for instance. Just to add to the fun, the uncertainty principle of quantum mechanics tells us that we can't really tell where a particle is or how fast it's moving at the same time.
Einstein wasn't having it. The idea that we couldn't really tell where a particle was or what is was doing must've been deeply unsettling to a physicist devoted to defining The Way the Universe Worked -- which Einstein did, with the theory of general relativity.
Now don't be frightened. General relativity has two big ideas: one about space and time, another about gravity. As you and I see it, space and time are in the background. They're fixed. They exist chronologically (and kind of monolithically.) In general relativity, space and time are one unified dimension (called space-time, conveniently). But here's the thing: Space-time may be big and unified, but it's not hanging out in the background. The theory of general relativity says space-time can be affected by matter. That means you -- as matter, existing -- are changing space and time.
OK, not exactly. It's actually really big things that are making space-time warp. The sun, for instance, is curving space-time toward it. And what would that imply? Ah, that's right: Smaller planets would fall into orbit around it.
Which brings us to gravity. Indeed, general relativity wasn't just Einstein patting Newton on the back and saying, "Yes, sir, gravity's a thing!" Instead, Einstein gave us a reason for gravity -- that the curvature of space-time made gravity exist, and made the universe act the way it did.
So what's the problem? Einstein showed us a mind-blowing way the universe works, and quantum mechanics shows us a fascinating way that particles on an atomic and subatomic level work. Unfortunately, one doesn't explain the other. Which means there must be some larger theory encompassing them ... or not?