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Universe glossary

Big Bang: The moment some 13.8 billion years ago when the universe began to expand from an almost infinitely dense and hot state.
cosmic microwave background (CMB): The microwave energy observed from all directions in the sky, at an equivalent temperature of 2.7 degrees above absolute zero, interpreted as the residual glow from the Big Bang.
dark matter: An invisible substance making up most of the mass in the universe that is detected by its gravitational influence. It has existed since the Big Bang.
dark energy: A mysterious pressure that is causing the universal expansion of the universe to accelerate.
galaxy: A massive, gravitationally-bound assembly of stars, planets, interstellar clouds of gas and dust, and dark matter.
gravity: The force of attraction between any two masses
mass: the amount of matter contained within a given object.
mass-energy: The combined total of the mass m and energy E of an object or region, related through Einstein’s equation E = mc2, where c is the speed of light.
matter: Anything that exerts gravity and moves slower than the speed of light. Visible, or normal matter is made of protons, neutrons, electrons, and other subatomic particles.
nuclear fusion: The combination of light atoms such as hydrogen and helium into heavier ones, such as helium, carbon, and oxygen. This process can release energy. Hydrogen fusion powers the Sun and other normal stars.
radiation: Energy that travels in the form of electromagnetic waves such as infrared, ultraviolet, or visible light, radio, X-rays, and gamma-rays, or in the form of subatomic particles moving near the speed of light.

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