重子不对称性是在物理宇宙学一个重要的问题，就是为什么在宇宙中，重子（重子是构成质子、中子等粒子）的数量比反重子多？根据在现在说明宇宙诞生的理论来看，粒子的数量应该和反粒子的数量一样多，而粒子会和反粒子湮灭产生光子(也就是电磁波)，因此宇宙应该是由完全电磁波构成的，而不会有任何的物质，但我们知道事实不是这样，因此出现许多的理论出来解释，其中可能是；宇宙有分许多不同的地区，有些地区是被物质占据，而其他的地区则是反物质，这些地区的之间建的距离很远，要不然不同地区的粒子就会互相湮灭，于是展开观察反物质的行动，但情况并不乐观，到2007年5月都没由任何比氦重的反原子核被观测到。因此这个问题还有待其他物理学家解决。

The baryon asymmetry problem in physics refers to the fact that there is an imbalance in baryonic matter and antibaryonic matter in the observable universe. Neither the standard model of particle physics, nor the theory of general relativity provides an obvious explanation for why this should be so, and it is a natural assumption that the universe be neutral with all conserved charges. The Big Bang should have produced equal amounts of matter and antimatter. Since this does not seem to be the case, it is likely some physical laws must have acted differently or did not exist for matter and antimatter. There are several competing hypotheses to explain the imbalance of matter and antimatter that resulted in baryogenesis, but there is as of yet no one consensus theory to explain the phenomenon.