So in the early 20th century a whole group of scientists (Bohr, Schrödinger, Heisenberg, Dirac and many others) made loads of progress with a theory to explain atoms and sub atomic particles and things like that. This is know as quantum theory, and it’s been developed into a few other areas with an absolutely astonishing agreement with experimental results.
Another group of physicists the biggest contributor of which was Albert Einstein worked on another theory, which became known as the General Theory of Relativity.
Now in physics we’ve managed to boil everything down to 4 forces. The electromagnetic force (this is responsible for an awful lot of the physics we see around us), the weak nuclear force (responsible for radioactive decay) the strong nuclear force (responsible for the stability of atomic nuclei) and gravity (responsible for apples falling on heads).
Quantum Theory neatly explains the first three. Really, really really well. In fact no experiment has really shown it to be wrong.
The General Theory of Relativity explains gravity, ridiculously well too.
Unfortunately it seems that the two theories never work out when used together, and it seems impossible to describe gravity using quantum mechanics, and (although I think less work is going into this) it seems that the other three can’t be incorporated into the General Theory of Relativity.
So they are working on something called superstring theory. Which might work out. If we find like 8 extra dimensions hiding somewhere.
Philip is right, we cannot describe gravity within quantum theory yet. There is also another problem with the quantum theory, and that is that in order to understand it we have to imagine that the observer is separate from the system. When we try to apply quantum theory to, say, the whole universe, we cannot do that because we are clearly inside the universe. Therefore we don’t really fully understand yet what our results mean.