Universe

The universe is space and everything in it.^[1][2][3][4] It is made of many billions of stars and planets and enormous clouds of gas separated by big spaces. The Big Bang started the expansion of the universe.

Astronomers use telescopes to look at distant galaxies. This is how they see what the universe looked like a long time ago. The past tense is because the light from distant parts of the Universe takes a very long time to reach us. From these observations, it seems the physical laws and constants of the universe have not changed.

Physicists are currently unsure if anything existed before the Big Bang. The size of the universe is not known.

1. ↑ Universe. 2010. {{cite book}}: |work= ignored (help)
2. ↑ "Universe". Dictionary.com.
3. ↑ "Universe". Merriam-Webster Dictionary.
4. ↑ Zeilik, Michael; Gregory, Stephen A. (1998). Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. ISBN 0030062284. The totality of all space and time; all that is, has been, and will be.