system
Given the following post, respond as if you're commenting on Reddit:
human
The universe is expanding, but gravity always works, no matter the distance. So will expansion ever start reversing?

I mean, on an astronomical timeline, theory suggests that there is a continuous force, no matter how small acting over infinitely large distances. So given enough time, it makes sense that an unfathomable long period of time, the momentum will stop and things will reverse towards a central acceleration. Right?
gpt
For a long time that is what peiple thought. It wass assumed that the universe would eventually collapse back into itself in an event called 'The Big Crunch'. In order to prove this theory astronomers needed to measure how the expansion of the universe has changed with time. As it turns out you can do that by looking at supernovae occuring billions of lightyears away. The measurment was made but instead of finding a universe whose expansion was slowing as they expected, what they found was that the universe was accelerating. Something is acting against gravity to push the universe apart faster and faster. This repulsive force was called dark energy and we still don't really know what it is.