What really is dark matter?

Dark matter is one of the Universe’s most mysterious and yet ubiquitous components. Humans, the Earth, the Sun, and everything that emits or absorbs light in space are all formed of ordinary matter, which includes protons, neutrons, and electrons. However, ordinary matter only accounts for one-sixth of the total mass in the Universe which leaves a vast majority an enigmatic part of our universe.

Over 80% of the universe’s matter is made of various materials that scientists have never observed. Almost 27% of this is called dark matter and we only presume it exists because the behaviour of stars, planets, and galaxies would be illogical without it. Over 70% of the unobserved materials account for what is called dark energy. This propels us to believe that what we experience is only a tiny fraction of reality.

Here’s what we know so far about the mysterious dark matter: 

There are no visible qualities of dark matter

This implies that dark matter is indeed dark. While normal matter consists of atoms that make up almost every visible thing in the universe like the stars, planets, and living organisms, dark matter is absent of any visible characteristics.

Because it emits no light or energy, ordinary sensors and detectors are unable to detect it. Its composition, according to scientists, is the key to its enigmatic nature. This could be attributed to the fact that dark matter does not react with any electromagnetic force which makes up the invisible-like property. Dark matter does not emit, absorb, or produce light or energy and as invisible as it is, it is actually believed to make up around 80% of the universe’s matter.

Dark matter isn’t just ordinary matter that we can’t see

Dark matter is not a black hole nor a failed star. It is likewise not clouds of gas or ordinary dust grains. Present cosmologic observations universally rule out this possibility. Thanks to the brilliance of scientists and the dependability of technology, we can actually calculate how much hydrogen, deuterium, helium-3, helium-4, and lithium-7 the Universe had right after the Big Bang based on the oldest, purest clouds of gas ever found.

These findings reveal that only one-sixth of the total mass required was identified to comprise the Universe. The remainder is something yet to be fully investigated and understood and is completely different from the celestial bodies and objects that are observable through the suite of evidence available: dark matter. It is believed to comprise the remaining five-sixths of the universe’s total mass.

The gravitational effect of dark matter is most evident where there is no normal matter

dark matter illustration

Scientists have long ruled out that dark matter is not just any normal matter that has turned dark. In fact, this is one of the most compelling pieces of evidence gathered about dark matter. When two galaxy groups or clusters collide, intergalactic gas and plasma clash and heat up generating X-rays. This is substantially more than what is seen in stars and individual galaxies, and it makes up the vast majority of normal matter, according to cosmologic measurements. The inevitable conclusion is that dark matter must account for the vast bulk of the Universe’s mass.

Dark matter does not interact with light or normal matter

There’s no denying that if dark matter exists, it had to emerge from somewhere in the early Universe. Since the data on dark matter is limited, scientists have asserted that dark matter seems to not interact with light or normal matter as any collision with these could result in the emission of particles that are detectable via direct observation.

Whatever the method was, those interactions that created dark matter are no longer occurring and haven’t done so in large numbers in a long time. Similarly, dark matter does not interact with light or normal matter in a way we would naturally expect.

Dark matter has yet to be discovered through direct detection experiments, limiting its mass and cross-section. It doesn’t absorb or filter distant starlight, which limits its light interactions. It does not annihilate itself above a certain threshold, or a massive and diffuse gamma-ray signal would be visible at galaxies’ centers. In fact, not communicating at all via any of these systems is 100 percent consistent. We’ll have to push these constraints considerably further if we hope to discover it directly, and even then, there’s no guarantee of a positive signal. Dark matter may not interact in these ways at all.

Dark matter must be cold

In theory, it is believed that dark matter must be cold in nature. The unidentified particle responsible for dark matter may have any mass and could have been produced moving at any speed relative to the speed of light.

Dark matter must be either very heavy or have been born very slowly. In other words, even in the early phases of the Universe, the dark matter had to be “cold,” rather than “hot” or “warm.” Currently, there is three observational evidence for this: the gravitational lensing of quadruply-lensed quasars, absorption characteristics along the line of sight to distant objects, and tidal streams in the Milky Way. These indicators constrain the temperature of dark matter.

The impacts of dark matter are most prominent, on average, in the tiniest galaxies

When a galaxy experiences a large burst of star formation, the radiation just passes through the dark matter when it would normally crash with and be absorbed by normal matter. This one is a little paradoxical, but it’s been confirmed by direct observation and treated similarly under gravitational rules. Other forces, such as nuclear and electromagnetic forces, however, only influence ordinary matter.

This makes dark matter exceptional and indicates that if your galaxy’s overall mass is low enough, normal matter can be evacuated by intensive star formation periods. The more normal matter is ejected from your galaxy as it gets smaller and less in mass, while all the dark matter remains.