Of the known fundamental forces, gravity stands out. Rather than being caused by force-carrying particles jumping between matter particles, gravity can be explained as the bending of space and time. In episode 13 of Subatomic Stories, Fermilab scientist Don Lincoln briefly sketches Einstein’s theory of general relativity — our current theory of gravity — and mentions some tests that prove that it’s right.
From CNN, April 6, 2019: Fermilab scientist Don Lincoln explains a result from the GRAVITY collaboration, which put Einstein’s theory of relativity to the test, making measurements of light near a black hole.
The inability of scientists to create a theory of quantum gravity arises from long-standing tensions between general relativity and quantum mechanics. There have been few approaches with any success. Don Lincoln explains one of the few promising ideas, called loop quantum gravity, in this 9-minute video.