Re: How does matter in the universe remain constant?

 i assume that when you say matter, you actually mean mass. the law of
conservation of mass is one of the four fundamental laws of physics.
Einstein showed that mass and energy are equivalent, so that if some mass
disappears, it is actually transformed into an equivalent amount of energy.

talking of reproduction, it appears that the cell seems to create something
out of nothing. but actually, it has to be fed with nutrients to
manufacture the new cell. so it is the mass of the nutrients that are
appearing in the new cells. 
A useful link