| Word | Meaning |
| Space | the boundless sky that stretches on without end |
| Universe | space, time, matter and energy taken as one whole |
| Celestial body | an object in the sky such as a planet, a star or a meteor |
| Observable universe | the part of the universe a telescope can reach |
| Light year (l.y.) | the distance light travels in one year |
| Astronomy | the science that studies the universe |
| Expansion of the universe | the continuous spreading out of the universe |
| Primeval atom | the extremely dense point from which all bodies and energy began |
| Prematter | the particles needed for matter to be built |
| Mathematical singularity | a point at which matter and energy are packed together beyond imagining |
| Dark matter | matter present in the universe that cannot be seen |
| Star cluster | a large group of many stars together |