The Universe is composed of two things, matter and light. Even these are only different manifestations of the same thing being able to be converted from one to the other.

The study of the composition and history of the Universe is called cosmology

Matter
Matter is all things with mass and volume. The term, "matter", is used to describe every physical substance in the Universe. Stars, gas nebulae, comets, planets, the ground we stand on, the water we drink and the air we breathe are all matter.

Matter comes in four states: solids, liquids, gases and plasma.

Light
Light is the other component of the Universe, it is not matter, but pure energy, known as electromagnetic radiation or EMR. Light travels through space at 300 000 metres per second. Known as the speed of light, this speed seems to be the fastest that anything can travel in our Universe. Matter can travel at speeds which approach the speed of light but never attain it. Only pure light energy can travel at "the speed of light"!

E = mc²
Albert Einstein described the relationship between matter and light, with his famous equation, E = mc². This equation equates energy and matter and states that the two are different manifestations of the same thing. That is, matter can be turned into energy in the form of light and light energy can be converted to matter!

Nuclear fusion reactions which occur in nuclear reactors and nuclear fission reactions which occur in the Sun and stars, generate energy from matter in accordance with Einstein's equation.

Converting light energy to matter is another matter (no pun intended) altogether, however, and may have only occurred in the original "Big Bang" when our Universes was formed.

Baryonic matter
Only in the last few decades have scientists started to think that there is more to "matter" than meets the eye. What we can see and feel, from the stars to bricks, is known as baryonic matter and may make up only 10% of the Universe because the rest, non-baryonic or "dark matter" - is invisible to us!

The baryonic matter with which we are familiar is composed of atoms which are themselves built up of subatomic particles called protons, neutrons and electrons.

Our knowledge of "dark matter" is still evolving and ranges from newly discovered "black holes" at the centre of our galaxy to neutrinos which travel near the speed of light and pass through the Earth and us as though we do not exist at all.

The study of matter in all its forms and the evolution of the Universe is called Cosmology, and is well worth further study.

The atomic and kinetic theories of matter
The modern atomic theory describes how matter is made up of extremely small and numerous particles called atoms. This theory has evolved over the past two millennia, from the early Greek theories of matter, through the dawn of the scientific era in the early 1800s to our present mathematical and scientific descriptions of matter.

Today, scientists believe that the many and varied properties of matter can be explained by the presence of atoms and combinations of atoms that form molecules and ionic crystals.

The second important theory of matter is "The kinetic theory". This theory describes how atoms interact to form the four physical states of matter: solids, liquids, gases and plasma.

The kinetic theory is used to explain the properties of the four physical states and the processes which occur during a change in state, like melting and boiling, in terms of the movement of particles and the forces of attraction between particles.