With a big telescope we can see millions of galaxies that are even billions of light-years away. But there is much more out there that we don't see.
Visible light is only a small part of the spectrum of electromagnetic radiation.
Most of the huge range of light rays arriving from the Universe is invisible to the human eye, and a large amount is blocked by the Earth's atmosphere. Yet astronomers have learned how to look at these invisible rays. With big radio telescopes on Earth and clever satellites in space they dramatically extended our knowledge about the Universe. And there will be many more discoveries in modern multi-wavelength astronomy. The shorter the wavelength, the higher is the energy of the radiation.