I'm a physics grad student and this is one of my favorite questions to explain. The key insight is that our everyday intuition doesn't always apply at extreme scales — very small (quantum), very large (cosmological), or very fast (relativistic).

Be careful about 'common sense' here — a lot of science is counterintuitive. The whole point of doing experiments is that the universe often doesn't work the way our gut tells us it should. The Earth feels flat and stationary, after all.

The math looks scary but it's mostly just a precise way of saying something you can understand in plain English. Once you get the concept, the equations are just bookkeeping. Focus on the idea first, the symbols second.

The honest answer is that scientists don't fully understand this yet. We have good models that make accurate predictions, but the 'why' underneath is still an active area of research. Anyone who tells you they have the complete answer is oversimplifying.