The actual matter density of the universe, accounting for all of the luminous matter and all of the dark matter known to exist in galaxies and clusters, is what fraction of the critical density?

 The correct answer is :  The critical density for the universe is approximately 10 ^ -26 kg / m ^ 3 or you can also say 10 hydrogen atoms per cubic meter.

Explanation:  This value is given by:

critical density = 3H ^ 2/2 Pi G, where H is the Hubble constant and G is Newton's gravitational constant.

Critical density is the average density of matter necessary for the universe to stop its expansion but after an infinite time.

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