As the mass of universe is not constant, it means that its linear momentum is not constant. But thinking in another way, is the universe subjected to a net external force? But what is external for the universe? The universe itself comprises of everything, so shouldn't every interaction be internal?
As every interaction is internal, there must be no external force, and the linear momentum of the universe must be constant!
Two different aspects give two different results. I am quite confused. Does the universe violate law of conservation of linear momentum?