system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we think that the universe should be slowing down without dark energy pushing it to accelerate?

How do we know that there is another force responsible for the acceleration of matter in the universe (i.e. dark energy) rather than the acceleration being a direct result of the big bang itself?  Why do we think that everything should be slowing down at this point without that force?  Having a bit of trouble with my phrasing as I'm not well versed, but hopefully this gets my question across.
gpt
Not that different than throwing a ball up in the air. The initial throw gives it some upwards speed, but as soon as you let go of it, it's gonna start decelerating because of its weight. You expect it to accelerate downwards for all of the jump. No matter how strong you throw it, it's always going to accelerate downwards all the time.

When the right equations are laid out in cosmology, a similar (but not identical) situation appears, where all the known energy density components (matter and radiation) work by decelerating the expansion. It doesn't matter how fast the expansion velocity was at the BB, it's always going to slow down in the sections where only matter and radiation are present. There is no "force from the BB" just like there is no "force from your hand" while the ball is in flight.

The fact that the expansion is accelerating right now therefore requires necessarily a driving component with exotic properties (in particular, a large negative pressure) and this would be dark energy and variants.