Question

what is dark matter
and
what is black hole

Answer 1

Dark matter is a mysterious substance composing most of the material universe, now widely thought to be some form of massive exotic particle. An intriguing alternative view is that dark matter is made of black holes formed during the first second of our universe's existence, known as primordial black holes.

Answer 2

Dark matter is a hypothetical type of matter distinct from ordinary matter such as protons, neutrons, electrons, and neutrinos.

Dark matter has never been directly observed; however, its existence would explain a number of otherwise puzzling astronomical observations.The name refers to the fact that it does not emit or interact with observable electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum.

Although dark matter has not been directly observed, its existence and properties are inferred from unexplained mass in gravitational lensing calculations, which affects the motions of baryonic matter and light.[4]. It influences the universe's large-scale structure, the formation of galaxies, and affects the cosmic microwave background.

The standard model of cosmology indicates that the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[5][6][7][8] Thus, dark matter constitutes 84.5%[note 1] of total mass, while dark energy plus dark matter constitute 95.1% of total mass–energy content. The great majority of ordinary matter in the universe is also unseen. Visible stars and gas inside galaxies and clusters account for less than 10% of the ordinary matter contribution to the mass-energy density of the universe. The most widely accepted hypothesis on the form for dark matter is that it is composed of weakly interacting massive particles, WIMPs, that interact only through gravity and the weak force.[14] The dark matter hypothesis plays a central role in current modeling of cosmic structure formation, galaxy formation and evolution, and on explanations of the anisotropies observed in the cosmic microwave background, CMB. All these lines of evidence suggest that galaxies, galaxy clusters, and the universe as a whole contain far more matter than that which is observable via electromagnetic signals.Many experiments to detect proposed dark matter particles through non-gravitational means are under way; however, no dark matter particle has been conclusively identified.

A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.

Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.


How Big Are Black Holes?
Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or "stuff," in an object.

Another kind of black hole is called "stellar." Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar mass black holes in Earth's galaxy. Earth's galaxy is called the Milky Way.

The largest black holes are called "supermassive." These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.