Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.

Source: https://science.nasa.gov

Since at least the 1920s, astronomers have hypothesized that the universe contains more matter than seen by the naked eye. Support for dark matter has grown since then, and although no solid direct evidence of dark matter has been detected, there have been strong possibilities in recent years.

"Motions of the stars tell you how much matter there is," Pieter van Dokkum, a researcher at Yale University, said in a statement. "They don't care what form the matter is, they just tell you that it's there." Van Dokkum led a team that identified the galaxy Dragonfly 44, which is composed almost entirely of dark matter.
Source:https://www.space.com/20930-dark-matter.html

In the very beginning, the Universe was incredibly hot and dense – so much so that a random local fluctuation that increased the density of an area by only 50 per cent would be enough to spark gravitational collapse into a black hole. Unlike the black holes we see in our modern Universe, the hypothetical ‘primordial’ black holes created at this time didn’t form from a collapsing star, which means they could have a wide range of masses, including very small ones.

Since they can be tiny and give out no light, primordial black holes are a potential candidate for dark matter, the mysterious material that makes up about 85 per cent of all matter in the Universe and can’t be seen through telescopes.

Shortly after the Big Bang, the expansion of the Universe accelerated. This phase of faster-than-light expansion is known as inflation.

During this period, it is theoretically possible for bubbles of ‘false vacuum’ to form – that is, bubbles of space that have a higher energy than the space that surrounds them. False vacuum bubbles are ‘metastable’, meaning that they are more or less stable, but, like a football balanced at the top of a hill, a small kick can knock them into a more stable, lower-energy state.
Source: https://www.sciencefocus.com

Dark matter works like an attractive force — a kind of cosmic cement that holds our universe together. This is because dark matter does interact with gravity, but it doesn’t reflect, absorb, or emit light. Meanwhile, dark energy is a repulsive force — a sort of anti-gravity — that drives the universe’s ever-accelerating expansion.

Dark energy is the far more dominant force of the two, accounting for roughly 68 percent of the universe’s total mass and energy. Dark matter makes up 27 percent. And the rest — a measly 5 percent — is all the regular matter we see and interact with every day.
In the 1930s, Swiss-born astronomer Fritz Zwicky studied images of the roughly 1,000 galaxies that make up the Coma Cluster — and he spotted something funny about their behavior. The galaxies moved so fast that they should simply fly apart. He speculated that some kind of “dark matter” held them together. 

Decades later, astronomers Vera Rubin and Kent Ford found a similar phenomenon when they studied the rotation rates of individual galaxies. The stars at a galaxy’s outer edge should circle slower than stars near the center. That’s the way planets in our solar system orbit. Instead, they noticed that the stars on a galaxy’s outskirts orbit just as fast — or faster — than the stars closer in. Rubin and Ford had found more evidence that some invisible form of matter is apparently holding the universe together.

“Even stars at the periphery are orbiting at high velocities,” Rubin once explained in an interview with Discover. “There has to be a lot of mass to make the stars orbit so rapidly, but we can’t see it. We call this invisible mass dark matter.”
Source: https://astronomy.com/news

In fact, the dark matter impact would generate so much heat that it would tunnel through body tissue as a flesh-melting plasma plume, the study authors reported.

That idea may sound far-fetched, but because dark matter can't be directly observed, seemingly outrageous proposals merit careful consideration until they can be conclusively ruled out, experts told Live Science. [The 11 Biggest Unanswered Questions About Dark Matter]

Most physicists hunting for dark matter are searching for particles smaller than atoms. But more-massive pieces of dark matter known as macroscopic dark matter, or macros, could lurk in the cosmos. In theory, macros could directly interact with physical objects such as human bodies, causing "significant damage," according to the new study titled "Death by Dark Matter."
Source: livescience.com