Why is it Dark in Space?

Our sky is blue during daytime hours because the light from the sun reflects off the gasses in our atmosphere.

It is blue due to the composition of the atmosphere. The sky on Mars has a red hue because its atmosphere

has a different composition that reflects mostly red light. Space is a vacuum (actually, the space between galaxies

contains about 1 hydrogen atom per cubic meter, and the space inside of galaxies contains about 1 hydrogen

atom per cubic centimeter), and there is nothing between the celestial bodies to reflect the light. We see other

planets, nebulae, moons, etc. because they reflect the light from other stars.

Current theory suggests the existence of Dark Matter, matter in space which does not reflect light. It is hypothesized that the total mass from all the visible objects in space does not account for the mass required to be in the universe, so physicists chalk the remaining matter up as dark matter.

"There is increasing evidence for the existence of large quantities of dark matter in the Universe.

The most direct evidence comes from the astronomical observation of the motion of visible matter

(stars and regions of neutral hydrogen gas) in galaxies."

- R. Flores and K.A. Olive, September 1989, Review of Particle Properties.

The Direct Detection Group at the CfPA

Looking for Dark Matter