The curious thing about matter is that if you zoom in far enough, you’ll find that it is energy, and energy is basically vibrations and waves rippling.

The way it’s often described, if you zoom into a hydrogen atom, it’s mostly empty space, let’s say the proton is the size of a tennis ball, the electron is like a grain of sand a kilometer away.

But then zoom further in, inside the proton, and it’s made of three quarks. But those three quarks make up only 1% of the proton’s total mass! The other 99% is like a boiling and roiling little sea of all sorts of particles popping in and out of existence, such as gluons (which keep those three quarks bound tightly together), as well as more quarks, and other more exotic particles, some of them heavier than the proton itself! But they disappear almost at the same instant in which they are created. This is a constant, non-stop process, happening inside every proton and neutron in the universe.

And what do you call that virtual stuff that always keeps on popping in and out of existence between the three permanent (aka valance) quarks and makes up 99% of the proton’s mass? That’s energy.

Now zoom back out, and back in, towards the electron; what’s going on there? It’s a lone particle (also a wave, but let’s not get into that now), there is no 1% / 99% of something, all valance no virtual.
But nudge that particle towards its’ antimatter counterpart, the positron, and both particles dissolve into a flurry of photons that instantly fly away at the speed of light - pure energy. It’s as if their little charge shells (electron - negative, positron - positive) dissolve and a flash of pure energy comes out.
This also applies to quarks and their antiquark counterparts.

That right there, is Einstein’s E=mc^2 in action, in all its’ glory. Energy equals mass.

Energy is something. It is there, even at the most fundamental and abstract level. It can be measured precisely, and explains so much so clearly, it’s incredible.

EDIT: fixed a sentence in the wrong place