Explaining String Theory
It's pretty simple really
To answer the original question, String Theory is not fact. That's why it's still called a theory. This is all very complicated stuff, but I'll try to put it simply.
Many years ago, the theory was that atoms were the tiniest particle that make up all things. Then scientists got out their high-powered microscopes and discovered that atoms are made up of smaller particles: protons, neutrons, electrons. Then they found out these things are made up of quarks, and so on.
It's gotten back to the point where we're no longer sure what the smallest particle could be. In fact, when you get into quantum physics, there are times where it's difficult to tell whether something is a particle at all. Particles sometimes act like waves, and waves sometimes act like particles. Scientists can even get particles/waves to act like one or the other depending on how they're observing it.
Long story short, String Theory states that the smallest unit of matter is a string, a tiny wave-like strand of energy too tiny for us to see, even with the most powerful electron microscope. These strands vibrate at different frequencies, and they form all the different things in the universe. Kind of like how a guitar string vibrates to create musical notes.
There's more to it, but that's the gist of it. I hope that helps.