What is mass? Where did stuff come from?
According to Einstein, mass comes in two forms: matter and energy. They are the same thing, sort of like ice and water, related by the iconic equation E=mc^2. (The ^ is a way to indicate a superscript when your keyboard can't do that).
A short, but significant digression-- c is the velocity of light and m is mass as matter, so mc^2, which is equal to mc*c, which has mc in it. mc is mass times a velocity which is what? Momentum!
E=mc^2 says that matter has energy. That's been pretty obvious since the last months of WWII. However, it also says that energy has mass just like matter.
This is not so obvious. When Einstein published his general theory of relativity, one of his results was that since light has energy, it must have mass, and therefore must be affected by gravity. (This is not how physicists usually put this, but it's still true in the context of this discussion.) A few years afterwards some other scientists showed that indeed, light rays are bent by gravity and by EXACTLY the amount that Einstein predicted.
But let's look at ordinary matter. The most obvious difference between matter and energy is that you can see matter. It's pretty solid and doesn't generally move unless something moves it. That's one of Newton's laws (I can't ever keep the numbers of those straight.)
Unfortunately, the closer you look at matter, the less solid and static it turns out to be. In fact, everything is always moving! If the molecules are not moving, they are certainly vibrating, and the electrons are going around the nucleuses (I mean nuclei), The neutrons and protons are vibrating, and who knows what the quarks inside them are doing, but you can be sure they're not napping.
In fact there is no such thing as matter at rest. This is weird.
To be continued...