Hello Krischna, we define their existence not only by light. First of all, it is all the electromagnetic spectrum that can be used to investigate stars and other celestial objects, but not less important is gravity and its effects. While at the first glance trivial, gravity effects (e.g. lensing) are used to determine stellar masses, categorize stars and planets, detect planets or other satellites and their properties and so on.
In general besides neutrinos, the fact that there is a lot of galactic radiation reaching our system, including nuclei, is used to analyse our galaxy. Gravity simulations are used to determine the evolution and current state of galaxies and the universe as a whole (cf. dark energy and dark matter). So light might be in the strictest sense the most obvious medium but it is not exclusive.
Thank you for your answer Volker. I would like to know about a few constantly existing components present in the electromagnetic spectrum regardless which object emits it. I am reading on the dark energy topics to relate and find if existing universal matter could also contain some dark matter. But not sure how they can be constant with stars, and other universal bodies.
Fermi's interaction constant G_F determines the strength of the interaction between a neutrino and an electron. It is a constant of nature and therefore does not change. In a way it is analogous to the Newton's gravitational constant G that determines the strength of the interaction between two masses.