One tricky question that plagued physicists for many years was...

How do matter particles interact?

The problem is that things interact without touching! How do two magnets "feel" each other's presence and attract or repel accordingly? How does the sun attract the earth?

We know the answers to these questions are "magnetism" and "gravity," but what are these forces?

At a fundamental level, a force isn't just something that happens to particles. It is a thing which is passed between two particles.