Dark Matter: The second highest stuff in the universe!
Dark matter is the elusive stuff in the Universe that interacts through gravity like ordinary matter, but it does not emit or absorb light. Dark matter contribution to the total energy density in the Universe is 26% while that of ordinary matter that is made of atoms is only 5%. The rest of the energy density, which is 69% is that of dark energy.
Astronomers detect dark matter gravitational influence but they do not see it. Imagine the challenges in understanding something that one cannot see. Billions of dark matter particles pass through each of us every second, yet no one notices because we cannot sense dark matter. Dark matter does not interact with light and dark matter is not made out of the same material as ordinary matter. It is not composed of atoms and elementary particles that we are familiar with.
When large amounts of dark matter aggregate into concentrated regions it's net gravitational influence is substantial, leading to measurable influences on stars and on nearby galaxies. Dark matter affects the expansion of the Universe, the path of light passing to us from distant galaxies, the orbits of stars and many other measurable phenomena in ways that convince us of its existence.
Dark matter might sound like an exotic suggestion to some, but proposing its existence is far less rash than revising the laws of gravity, as dark matter skeptics might prefer. Dark matter is likely to have a more or less conventional explanation that is completely consistent with all known physical laws. After all, why should all matter that acts in accordance with known laws of gravity behave exactly like familiar matter? Why should all matter interact with light? Dark matter can simply be a matter that has different or no fundamental charges. Without electric charge or interactions with charged particles, the dark matter simply cannot absorb or emit light.
The main candidate for dark matter is a new elementary particle called WIMP ( weakly interacting massive particle). Dark matter is of two kinds: cold (CDM) or hot ( HDM) depending on its velocity. Pif the particle velocity is slow compared to that of light then it is referred to as cold dark matter. However, if it moves with a velocity close to that of light then it is referred to as hot dark matter. The main candidate for HDM is the neutrino.
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