Something led to a tiny imbalance in the matter/antimatter numbers at the beginning of time, with matter slightly in the lead. That imbalance — called the matter/antimatter asymmetry — gave us the leftover matter that forms everything in the universe, from galaxies and hydrogen clouds to you and slime moulds.
Finding the cause of the imbalance means finding a brand new bit of physics — like a new particle or force — and it’s one of the more exciting and challenging jobs in particle physics right now.
So how do you look for the mysterious factor that led to our leftover matter?
You look at how matter and antimatter behaved in the ridiculously high energy environment straight after the Big Bang, and see if there’s any oddness that could account for the tiny excess of matter needed to explain all the matter that’s here today.