According to optics, there are two types of matter on the basis of emission of light–luminous and non-luminous. Those matters which emit visible light are luminous and those matters which don’t emit light are non–luminous. This means that luminous matter emits light that fall within the range of visible wavelength.

There may be three possibilities for any object to be non–luminous - either that doesn’t emit light or that emit light which lies off the visible wavelength or that matter is made up of those fundamental particles  which is not composed of visible matter coined as dark matter.

From the classical law of gravitation and from the General Theory of Relativity (GTR) the critical density of the universe is found to be four hydrogen atoms per cubic centimeter. Critical density of the universe is the limiting theoretical density. If the density of the universe is more than it, then it will not expand forever due to substantial gravity, and, if it is less than it, then the universe will expand forever because of weak gravity.

So the estimation of the average density of the universe is done by many techniques. Among them, one way was by counting the galaxies in the patch of the sky and another way was motion of the galaxies in the cluster of the galaxy (rotational curve).

The first technique revealed the density of the luminous matter of the universe to be 4% of the critical density and the second technique revealed the density of the all (seen and unseen) matter to be 26% of the critical density. This shows that most of the matter in the universe is unseen, and called dark matter.

Since the density of the universe is seen to be less than the critical density, this shows that the universe would be expanding forever. In the terms of Physics, it can be said that the farther galaxies would have more red shift than the nearer ones. But the result was surprising, it just contradicted our educated guess!

It means that the universe is speeding out. Astronomers and theorists suggested that the space is suffused with a kind of energy that has no gravitational effect and emits no electromagnetic radiation, but acts like anti–gravity repulsion. This invisible, immaterial energy is called dark energy. Observation shows that the density of the dark energy is 74% of the critical density. It implies that the main portion of the universe is mysterious containing dark matter and dark energy.