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The Universe begins, "preposterously" Glossary

• Atom: the smallest unit of an element, having all the characteristics of that element and consisting of a dense, central, positively charged nucleus surrounded by a system of electrons.
• Brown dwarf star: a theorized star (too dim to detect) that has 0.08 % the mass of our Sun. It's too small for nuclear reactions to begin.
• Dark energy: Unknown energy that may make up 65 % of the Universe. The energy is repulsive and shoves galaxies away from each other at an ever-increasing speed. Dark energy only shows up over large fractions of the observable Universe.
• Dark matter: Unseen matter that may make up more than 30 % of the Universe. As the name implies, dark matter does not interact with light or other electromagnetic radiation, so it cannot be seen directly, but it can be detected by measuring its gravitational effects. It is believed that dark matter was instrumental in forming galaxies early in the Big Bang.
• Electron: A subatomic particle in the lepton family having a negative electric charge.
• Fundamental forces: The four basic forces of nature are the familiar gravitational force, the electromagnetic force, the strong force and the weak force (which is responsible for radioactivity). These forces act between elementary particles--the basic building blocks of matter. All visible matter and its behavior can be described in terms of these particles and the forces that act between them.
• Lepton: A lepton is any of a family of elementary particles that participate in the weak force, for example, the electron.
• Microwave radiation: A high-frequency electromagnetic wave, one millimeter to one meter in wavelength, between infrared (heat) and short-wave radio wavelengths.
• Photon: A bit of light---the quantum of electromagnetic energy--- a discrete particle having zero mass, no electric charge, and an indefinitely long lifetime.
• Polarization: A state in which light-wave vibrations line up in the same direction, like the wave created on a jump rope when jerked sideways or the glare off a lake blocked by polarizing sunglasses.
• Quark: Quarks are any of a group of six point-like (almost without size) elementary particles. Quarks make up other particles, except leptons. A proton, for instance, is one 'down' and two 'up' quarks. However, an electron isn't made up of quarks because an electron is a lepton.