Matter is any substance that has mass and takes up space. Earth, and everything on it, is made of matter, so are all the stars and planets in the universe. According to the Big Bang theory, at the beginning of the universe, all matter erupted from a singularity, a hot, dense speck of matter.

The most familiar forms of matter—elements and compounds—are made of up tiny particles called atoms. Atoms themselves are made up of even smaller bits of matter: protons, neutrons, and electrons. Electrons, which orbit an atom’s nucleus, are considered fundamental particles because they cannot be broken down further. Protons and neutrons, however, are made up of smaller particles called quarks.

All of these particles are forms of matter because they have mass and occupy space. Light, which is made up of particles called photons, is not considered matter because it has no mass. However, it does carry energy, which can be transfered between atoms.

Matter can exist in different states, or phases. For example, water exists on Earth in three states: solid, liquid, and gas. In each of these states, the water molecules are identical—water’s chemical properties do not change. However, the different states of water do have distinct physical properties.

When water is in the solid state—ice—its atoms stick together rigidly and only vibrate. Ice keeps its shape no matter what container it is put in. When ice melts and becomes liquid, the water molecules move freely, though they still interact with one another. Liquid water will spread out to fit whatever container it is put in, limited by the amount, or volume, of water. When this water evaporates and becomes a gas (water vapor), the molecules move around quickly and randomly. Gases, such as water vapor, will expand to fill whatever container they are put into, regardless of volume.

Water changes state when the temperature or pressure around it changes in the right way. If you add enough heat to an ice cube, it will melt. This is because with more energy, the water molecules move faster and more freely. Add enough energy to liquid water and the molecules can escape as water vapor.

There are other states of matter that do not occur frequently on Earth because they only happen with temperatures and pressures so extreme they rarely happen on this planet. One of these states is called a plasma. A plasma forms when a gas is heated so much that electrons escape from atoms and move around freely. Plasmas make up the sun and stars, and may be the most common state of matter in the universe.

An ongoing mystery for scientists concerns antimatter. Every kind of matter particle has a corresponding kind of antimatter particle with the same mass but opposite charge. The antimatter counterpart of an electron is a positron, a positively charged particle with the same minuscule mass (about 1,800 times smaller than a proton). When a matter particle and its antimatter opposite collide, they “annihilate” in a burst of pure energy. Antimatter exists in very small quantities in our universe today. Antimatter particles are produced by some kinds of radioactive decay and they are created artificially in particle accelerators.

However, studies show the earlier universe produced equal amounts of matter and antimatter. So where did all the antimatter go? The matter and antimatter should, in theory, have canceled out, producing a sea of light with no matter remaining. And yet, a matter-filled universe exists today. Many physicists have dedicated their lives to solving this cosmic riddle.