system
Given the following post, respond as if you're commenting on Reddit:
human
Is The Mass of The Universe Constant?

If not, how can it be gained/lost?
gpt
Other comments suggest mass itself is not conserved while mass+energy is. It's true mass and energy are interchangeable but in general relativity they are not conserved on a large scale. Conservation of energy is consequence of time invariance in physics, but under GR different reference frames variate in time significantly. 

An example of this is how light, when traveling through interstellar space, is redshifted. The light lowers in frequency and looses energy as it moves through expanding space - this lost energy doesn't go anywhere, it is just lost.