31.3 and 31.4

Question 1

Galaxy

Answer 1

a huge collection of gas, dust, and billions of stars and their solar systems. A galaxy is held together by gravity.

Question 2

milky way

Answer 2

a large barred spiral galaxy.

Question 3

Local Group

Answer 3

Local Group, in astronomy, the group of more than 20 galaxies to which the Milky Way Galaxy belongs.

Question 4

Cosmology

Answer 4

the science of the origin and development of the universe. Modern astronomy is dominated by the Big Bang theory.

Question 5

Big bang theory

Answer 5

massive explosion that not only created the majority of matter, but the physical laws that govern our ever expanding cosmos.

Question 6

Cosmic background radiation

Answer 6

in every direction. It is thought to be residual radiation from a period shortly following the big bang, when the universe was very hot. Also called microwave background.

Question 7

Dark matter

Answer 7

nonluminous material that is postulated to exist in space and that could take any of several forms including weakly interacting particles

Question 8

Dark energy

Answer 8

a theoretical repulsive force that counteracts gravity and causes the universe to expand at an accelerating rate.