In the universe, there are various entities or phenomena that do not emit or absorb light waves, at least in the visible spectrum. Here are a few examples:
Dark Matter: Dark matter is a hypothetical form of matter that does not interact with electromagnetic radiation, including light. It is believed to make up a significant portion of the total mass in the universe. The exact nature of dark matter is still unknown, but it is thought to consist of particles that interact primarily through gravity.
Black Holes: Black holes are regions of spacetime with extremely strong gravitational forces that nothing, including light, can escape from within a certain boundary called the event horizon. As a result, they do not emit or reflect light. While black holes themselves are not made up of anything in the conventional sense, they are formed from the remnants of massive stars that have undergone gravitational collapse.
Neutrinos: Neutrinos are subatomic particles that have a minuscule mass and no electric charge. They interact extremely weakly with matter, including electromagnetic radiation, making them difficult to detect. Neutrinos are produced in various astrophysical processes, such as nuclear reactions in the Sun or supernova explosions.
Cosmic Microwave Background (CMB): The cosmic microwave background is the remnant radiation from the early universe, emitted when the universe was about 380,000 years old. It is a faint radiation that permeates the entire universe and has cooled down to microwave wavelengths over time. While it is not completely devoid of light, it is in the microwave region of the electromagnetic spectrum and is not visible to the human eye.
These entities or phenomena have different properties and are "dark" in the sense that they do not emit or interact with light waves in the visible spectrum. The reasons behind their lack of interaction with light vary. In the case of dark matter, its exact composition is unknown, but it is postulated to consist of particles that have weak or no electromagnetic interactions. Black holes have such intense gravitational forces that not even light can escape them. Neutrinos have extremely low mass and interact very weakly with matter, including photons. The cosmic microwave background is a relic of the early universe, and its low-energy photons have shifted to microwave wavelengths over billions of years.