The nature of gravity - and whether it can be reconciled with quantum mechanics - is one of the biggest mysteries in physics. Most researchers think that at a fundamental level, all phenomena follow the principles of quantum physics, but those principles do not seem to be compatible with the accepted theory of gravity. For years, researchers have been proposing experiments to show whether gravity could produce a phenomenon known as quantum entanglement.
This tremendously successful theory gave us everything from gravitational waves to black holes based on one profound insight: that the fabric of spacetime itself would evolve, curve, and even ripple based on the properties and behavior of the matter and energy within it. When we applied Einstein's equations to the entire Universe as a whole, along with the idea that the Universe was filled nearly uniformly with matter and energy on the largest scales, we wound up with an expanding Universe.