It’s common knowledge that whenever two opposites meet, they cancel out. When antimatter and matter meet, they don’t simply “cancel out” - They both explode with an efficiency of 100% (meaning that 100% of the mass is converted into energy.) For reference, when a nuclear bomb is detonated, only 2% of the total mass is converted into energy, and the rest becomes gas, smoke, and solid waste. The explosion from simply 1 gram of anti-matter is roughly equivalent to 1 tsar bomb. The tsar bomb’s yield was equal to 50 megatons of TNT - That's 1400 times the Hiroshima and Nagasaki bombs combined… So we know for a fact that if antimatter were to exist abundantly in the universe today, the results would be disastrous.
According to the Standard Model, antimatter and matter should have been created in equal parts. The Standard Model is a model of the universe and how it’s particles interact, and it’s the most accurate theory we have so far. So if antimatter and matter were created at the same time, in the same amounts, where is all the antimatter? There isn’t really an answer to this question, but we can speculate. A fairly straightforward reason for this could be that by luck, antimatter and matter emerged from opposite sides of the point of singularity, and so they never touched. This means that if you travelled in a straight line for a long, long, time you would eventually come across an anti-matter universe. How likely is this? Not very. The sheer probability of antimatter and matter being on the opposite sides of each other within the singularity is very small.
Another explanation for this is that the initial explosion that started expansion, the big bang, was of such a high intensity and speed that particles right next to each other did not have time to react, and the amount of heat was so high that everything expanded extremely fast, without actually touching. This then caused pockets of antimatter within groups of matter, and different antimatter concentrations throughout the whole universe. Particles react with each other in a very short time, but considering the fact that the big bang’s momentum is still moving forward 13.8 bullion years later, it’s not too far from logic to imagine that such conditions might have been present at the time. This is the theory that is the most likely, and also one that is widely accepted.
But what if we’re wrong? We don’t actually know we’re right. All we have are theories that we don’t have the tools to confirm. Yet. For years to come the riddle of antimatter and matter will remain unsolved, however it’s lesser significance will not hinder scientific progress in other areas in the field of Astronomy.