Final answer:
Determining the density of the universe is challenging but astronomers use methods like counting galaxies and estimating their masses to estimate the average density of matter. The estimated density is too small to stop the expansion of the universe.
Step-by-step explanation:
The density of the universe is a complex topic due to the difficulties in determining it accurately.
Astronomers use various methods to estimate the average density of matter in space, such as counting galaxies and estimating their masses including dark matter.
These estimates indicate a density of about 1 to 2 × 10-27 kg/m³ (10 to 20% of critical density), which is too small to stop the expansion of the universe.