Scientists
calculate the mass of large objects in space by studying their motion.
Astronomers examining spiral galaxies in the 1950s expected to see material in
the center moving faster than on the outer edges. Instead, they found the stars
in both locations traveled at the same velocity, indicating the galaxies
contained more mass than could be seen. Studies of the gas within elliptical
galaxies also indicated a need for more mass than found in visible objects.
Clusters of galaxies would fly apart if the only mass they contained were
visible to conventional astronomical measurements.

The dark matter is not antimatter, because we do not see the unique
gamma rays that are produced when antimatter annihilates with matter.
Finally, we can rule out large galaxy-sized black holes on the basis of
how many gravitational lenses we see. High concentrations of matter bend
light passing near them from objects further away, but we do not see
enough lensing events to suggest that such objects to make up the
required 25% dark matter contribution.

However, at this point, there are still a few dark matter possibilities
that are viable. Baryonic matter could still make up the dark matter if
it were all tied up in brown dwarfs or in small, dense chunks of heavy
elements. These possibilities are known as massive compact halo objects,
or "MACHOs". But the most common view is that dark matter is not
baryonic at all, but that it is made up of other, more exotic particles
like axions or WIMPS (Weakly Interacting Massive Particles).
Source : academic writing services

Dark matter is a kind of matter in astronomy and cosmology to account for gravitational effects that appear to be the result of invisible mass. Dark matter cannot be seen directly with telescopes; evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level. It is otherwise hypothesized to simply be matter that is not reactant to light. Instead, the existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. Although dark matter makes up most of the matter of the universe, it only makes up about a quarter of the composition. The universe is dominated by dark energy.

Dark matter is a concept in particle physics that suggests that there is matter in the Universe which is not visible but greatly influences observable matter. Dark matter makes up approximately 20% of the Universe and is 5 times heavier, in mass, than all of the stuff that scientists can observe (such as stars, planets, asteroids, galaxies etc.). However, scientists are not able to observe or interact with the vast amounts of dark matter because dark matter has no electro-magnetic interation: it doesn't reflect or give off light. The reason we know that dark matter exists is because of 2 main reasons:

1. Matter doesn't spin the way they should; stars rotate faster than allowed by visible matter. This shows that there is other matter in the Universe with great mass, hence great gravity, allowing the stars to spin faster.

2.The other way we can "see" dark matter is through "Strong Lensing".

Let's say that you are looking at a galaxy through a microscope and there is dark matter between you and the galaxy. Light comes from the galaxy from one way ie. The left side of the dark matter and is bent by the gravity of dark matter and enters your eyes: you see a galaxy on the left side relative to the dark matter.

However, Light also comes from the other direction ie. The right side, and, similarly is bent by the dark matter's gravity and enters your eyes: you see the same galaxy on the right side relative to the dark matter!