Ever since the creation of the universal law of gravity by Isaac Newton, there was a lot of controversy regarding this, perhaps the most familiar phenomenon in all physics. Newton himself never knew what gravity really was and it boggled him, even though he could describe how it works. When Einstein develop the general theory of relativity which answered a lot of questions about gravity, but at the same time a lot of new questions were put forward. However, gravity became a real outcast when quantum mechanics was highly developed in the second part of 20th century. This lead to unification of electromagnetic, weak and strong nuclear forces, however, gravity just couldn’t be unified with these forces.
The famous equation of gravity by Isaac Newton
One of the approaches to solving this problem was describing gravity as an emergent force, and even though a lot of work has been done with these sort of approaches, no emergent theory of gravity that works currently exists. One of the promising new theories is by a guy named Erik Verlinde, who discuses his approach in this Big Think video.