It is a hypothetical type of matter. It does not emit with electromagnetic radiation. Light emit with electromagnetic radiation, so we can see light in electromagnetic spectrum. But we cannot see it. It is invisible in electromagnetic spectrum.
Dark matter has not observed directly. It observed by gravitational effects on visible matters like gravitational lensing.
Dark matter is transparent for electromagnetic radiation. It is so dense and small that fails to emit radiation . Thus we cannot detect it by our known imaging technology.
According to The Standard Model of Cosmology total mass-energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. That’s means that all stars , galaxies and matters which we can observe and fell that is only 5% of whole universe.
Dark matter plays a central role in cosmic structure formation, galaxy formation and evolution. It composed with Weakly Interacting Massive Particle (WIMPs) that interacts with only through gravity and weak force.
Most evidence of the it comes from the motions of the galaxies. According the Virial Theorem, the total kinetic energy should be half of the galaxie’s total gravitational binding energy. But observationally, the total kinetic energy is greater than gravitationally binding energy. So this means that there will be an invisible matter that have more mass. Law surface brightness dwarf galaxies are important source for studying it.
There is one another big reason to predict the dark matter. In the beginning (before 1998), we think that universe stopped to expansion and it start to slow to accelerate due to gravity. The universe is full of matters and gravity attracts that all matters together. After then, In 1998 the Hubble Space Telescope showed that our universe was expanding slowly that it is expanding today. That means universe is expanding fast today than past. So it is accelerating. So after then many theories come.
Einstein also give his theory and he give a ‘cosmological constant’. So, off course space is not space. It contains many matters and energy that we cannot detect and observe it. Dark matter and dark energy both are hypothesis yet.
May be the dark matter made of baryonic matter or non-baryonic matter.