Scientists Say: Dark matter
Dark matter (noun, “DARK MA-ter”)
Most of the stuff in the universe that occupies space and has mass is not matter we can detect. It’s dark matter — a physical object or particle that appears to emit no radiation. Scientists aren’t completely sure dark matter exists. But they know something in the universe is affecting visible objects with its gravity. They’ve given that something the name dark matter.
Scientists estimate that normal matter — stars, planets and other objects we can detect, including ourselves and everything else on Earth — makes up only 5 percent of the universe. Dark matter makes up 27 percent. The rest is a mysterious force called dark energy.
In a sentence
An Earth-orbiting telescope has picked up a new X-ray signal that might be a sign of decaying dark matter.
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dark energy A theoretical force that counteracts gravity and causes the universe to expand at an accelerating rate.
dark matter Physical objects or particles that emit no detectable radiation of their own. They are believed to exist because of unexplained gravitational forces that they appear to exert on other, visible astronomical objects.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
matter Something which occupies space and has mass. Anything with matter will weigh something on Earth.
particle A minute amount of something.
physical (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
universe The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).
X-ray A type of radiation analogous to gamma rays, but of somewhat lower energy.