According to this site , there might be more matter in the universe because:
Do physicists understand why the weak force would favor matter over antimatter? Can anyone make me understand? 
The slightly flippant answer is that matter is favoured over antimatter because if it wasn’t we’d call antimatter matter and vice versa. That’s not quite entirely flippant because it is important to realise that the distinction between the two is ortherwise arbitrary.
All the observed non-gravitational interactions between sub-atomic particles are consistently described by the Standard Model. Underlying it are a small number of physical symmetries that severely restict the maths involved. Those are regarded as deep principles that determine whether or not particular particles can change into others. This Standard Model is now very well supported by experimental evidence; indeed there’s essentially nothing out there that explictly contradicts it, despite three decades of looking for it.
But the thing is that, despite all the other restrictions they impose, the symmetries involved don’t force the weak nuclear force to be the same depending on whether it’s acting on matter or antimatter. And so, because it isn’t disallowed by the underlying symmetries, the Standard Model allows for the effect to be different for the two.
Which is what is observed in laboratory experiments. Extrapolating from those experiments and, more generally, the Standard Model allows physicists to handily account for the observed excess of matter over antimatter.
So nobody knows quite why the difference is the size it is, but - given the Standard Model and its ability to pretty much explain everything else - it’s unsurprising that there’s some amount of difference. The hope is that some better future theory will explain the observed amount of difference.