system
Given the following post, respond as if you're commenting on Reddit:
human
Assuming the universe is not a perfect vacuum, is it ever possible that a moving object will slow down to rest without encountering another object?

From what I understand, the universe is not a perfect vacuum, and there are random bits of atoms floating around space. Overtime, would enough of these random atoms be able to slow down a moving object?

Edit: I think I made a mistake, I meant that "outer space" is not a perfect vacuum.
gpt
Here's an order of magnitude estimate:

The average density of the universe is approximately one proton per cubic meter.  So, a 1 meter sphere will hit a proton every meter it travels.  In 10 billion years, at nearly the speed of light, it will encounter about 10^(26) protons (about 160 grams of protons).  If this object has any appreciable mass (e.g., a 1000 kg rock) it will not slow down much due to collisions.  

Beware:  This estimate depends on the size of the projectile.  Small objects are affected more.

A much larger effect is the red-shift (due to the Hubble expansion) that this object will endure.  Its speed with respect to the local matter will approach zero.  Yes, everything (not only photons) is subject to the red-shift, which is a geometrical effect.