A huge part of the matter in the universe is: dark matter. You can't really "see" it. But, you could see the effects of its gravity. Experts are thinking about how this matter can really behave.
Scientists can see how dark matter is distributed based on how its gravity affects light. However, when astronomers compared recent data from the Hubble Space Telescope and the Very Large Telescope to current models, something didn’t add up.
"Current assumptions" about dark matter physics might not be entirely correct. Watch the video for more explanations.
Dark matter is a form of matter thought to account for approximately 85% of the matter in the universe and about a quarter of its total mass - energy density or about 2.241×10⁻²⁷ kg/m³.
Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter, such as protons or neutrons.
Here is the explanation of "dark matter" in really simple terms. Dark matter is composed of particles that do not absorb, reflect, or emit light, so they cannot be detected by observing electromagnetic radiation. Dark matter is material that cannot be seen directly. We seem to know that dark matter exists because of the effect it has on objects that we can observe directly.
Proving dark matter is a difficult task. Scientists have not yet observed dark matter directly. It doesn't interact with baryonic matter and it's completely invisible to light and other forms of electromagnetic radiation, making dark matter impossible to detect with current instruments.
Some may wrongfully think that there is no dark matter. However, without dark matter, galaxies would lose a large fraction of the gas that forms new stars immediate after the first major star-forming event they experienced.
Dark matter theory is certainly needed to account for the fact that galaxies don't seem to obey the fundamental laws of physics. That led scientists to believe there must be some invisible matter there to create a stronger gravitational pull and faster stellar motion.