Interstellar and Intergalactic Space:
The space between the stars and the galaxies looks empty, doesn't it? But this 1s not true. In the great dark between stars in the galaxies, and galaxies in the clusters, there are clouds of gas and dust. The gas clouds are mainly made up of hydrogen atoms and cannot be seen by the unaided eye. Only the modem astronomical instruments have been able to detect these particles. Cosmic dust is made up of bigger particles. These clouds of dust are revealed when they reflect the light of stars falling on them. Cosmic dust and clouds of gas in a galaxy are found to play a great role in the formation of a star.
In the gas clouds and cosmic dust we also find traces of different kinds of ordinary molecules, like water, ammonia, carbon monoxide etc. There are many organic molecules, like msthane, methanol (also known as wood alcohol), formic acid (the substance that gives ant and bee bites their sting), and many more. The organic molecules are the matter out of which our kind of life arose on the planet Earth. The abundance of such organic molecules in the interstellar space suggests that there might be life somewhere out there, perhaps in a different form. We may not be the only ones, after all!
Another major constituent of the universe are cosmic rays. Cosmic rays are beams of charged particles, such as the electrons, protons and helium nuclei etc., that freely travel in space at nearly the speed of light. These particles carry large amounts of energy across space. Let us now end our brief journey of the universe, and summarise what we have discovered. The matter of the universe is concentrated in large superclusters of galaxies, each measuring 100 to 200 million light years across and each containing millions of galaxies. The galaxies are grouped in rich or poor clusters, have different shapes and are distributed in different ways. The galaxies contain stars and clouds of gas and dust. Stars may be grouped in clusters or they may be individual stars like the Sun having planetary systems like our Solar System. Our planet Earth is a part of the Sun's family.