system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is The universe "flat"
gpt
This doesn't literally mean the universe is flat like a piece of paper; it's not a literal description of the universe in the way we understand the word "flat".

A spherical universe would be like the Earth: if you go in any direction for long enough, you would eventually end up back where you began. In a spherical universe if you flew in any direction you would eventually end up back in Earth.

Evidence suggests that this is not the case. Look at a map, not Google Earth, but a real map. If you put your hand on Ghana and keep walking east, you're going to walk right off the map and onto the table, and you're going to keep walking until something stops you.

A flat universe means that the universe doesn't have a curve to it that would result in you ended up back where you began, which would certainly happen if you completed a circumference of the Earth. Whether or not this means the universe is truly infinite is still a matter of debate, but the consensus is that the entire universe is infinite but the observable universe is very much finite; after all, the speed of light is the fastest you can travel in space, but the "speed of space", i.e. the rate at which the universe expands is much faster, therefore the unobservable invisible part of the universe that cannot be observed because it is older than light and is further away than light can reach us from, is considered effectively infinite as we have no means of establishing its size.

Think about that using this method: imagine you cannot see, and your knowledge of reality only extends to how far you can hear. You are confined to your home, so the furthest you can hear is the road outside. This is the absolute limit to your knowledge, but does it mean that there is nothing beyond?

A universe that is truly flat, i.e. truly infinite in every definition of the word, would have infinite energy, thus infinite mass. Think of it like a procedurally generated video game: no matter how far you go, there is no limit to what you can discover. 

A spherical universe would have finite energy and thus finite mass, so there is a fixed and undeniable absolute maximum to how much the universe can support. A natural process of the universe is entropy, which is the natural decay of the universe caused by energy being lost. In an flat universe, entropy could be reversed because there is infinite energy, so electrons and protons breaking down would be like sand being washed from the beaches and ending up somewhere else: the energy survives. In a spherical universe, this would not be the case: energy that is lost is lost forever, and there is an inevitable and inviolable fate where the universe will die.