system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the universe is expanding, but the amount of matter in it remains constant(ish), does that mean the 'average density of the universe' is decreasing?

Not sure this question makes a ton of sense period, let alone from an actual physics standpoint. But in general terms, is this a valid question and if so, what's the answer and its effects?
gpt
Yes, the universe is becoming more diffuse, in mass and in energy. If you measure energy density as a temperature, it's only a few degrees (Kelvin) on average now. It dropped very rapidly during the big bang and initial inflation of the universe and has been declining more slowly ever since.