The physical universe consists of:
A. Matter and B. Energy.
A. Matter is of two kinds:
(a) Ordinary matter we are familiar with, such as water, sand, iron, air, etc. This matter is ultimately made up of molecules which consist of atoms. More on these when another question is asked about atoms and molecules.
(b) Dark matter. This is a kind of matter about which we know as yet very little. Scientists assume that there is some kind of matter which we have not yet been able to detect, but which is exerting some (gravitational) force on the galaxies (large collections of billions and billions of stars). Astronomers say that almost 85% of the total matter in the universe is dark matter.
B. Energy is of two kinds:
(a) Ordinary energy. This is in various forms: heat, light, sound, x-rays, etc. (radiation) and motion of bodies.
(b) Dark energy: Like dark matter, we know very little about dark energy. We assume there is such a thing in order to explain why galaxies are all moving away from one another at every increasing speeds.
According to astronomers the universe consists of:
26.8% dark matter, 68.3% dark energy, and 4.9% ordinary matter-energy.
An ancient Tamil poetess called Auvaiyaar said:
What we known is like a handful of mud; what we don’t know is as vast as the universe.
Modern astronomers say: The matter and energy that can be observed is like a handful of mud; what can’t be observed is as vast as the whole universe.