What is the significance of each of the following in the study of astronomy: (a) Dark Matter (b) Dark Energy (c) 21 cm Radiation

Dark matter - This are the invisible bodies of rocks and other objects that exist on the space and are proved by the scientist using various calculating method. They do not emit or reflect light but their presence can be identified due to the effects of gravity on the galaxies.

Dark energy - The energy that drives the universe to become large and expand is known as the dark energy. It has been found to exist since the start of big bang.

21 cm radiation - This are radiations of wavelength equals to 21 cm that are released by the cold atomic hydrogen.