There are very good reasons to believe that at the time of the Big Bang, some 13.7 billion years ago, matter and antimatter were created in equal amounts. But the Universe as we see it today is made almost entirely of matter. It is known that when the Universe was about 380,000 years old, matter and antimatter had annihilated producing what we now see as the cosmic microwave background radiation, but a small amount of the original matter was left over, and this makes up the matter of the Universe as we now see it. Thus the laws of nature are not symmetric between matter and antimatter; somehow, before the Universe was 380.000 years old, some of the antimatter had converted to matter, but this was not balanced by an equal conversion of matter to antimatter.
The Standard Model of particle physics includes a mechanism, known as CP-violation, that can induce the sort of matter-antimatter imbalance described above. This mechanism has been studied and verified in many particle physics experiments. The problem is that the effect of the Standard Model CP-violation is far too small to account for the original imbalance in the early Universe. There must be some other mechanism that induces matter-antimatter asymmetry, and particle physicists are searching for it. There could be something in the quark sector that is not yet understood, or the sought-after mechanism may be in the lepton sector and related to the behaviour of neutrinos.