What encompasses everything that exists, including all matter and energy?

The universe refers to the totality of everything that exists, including all forms of matter, energy, planets, stars, galaxies, and even the vast spaces in between. It is the largest and most comprehensive term used in cosmology, representing all physical laws and realities. This definition extends beyond individual galaxies or solar systems, which are smaller components of the universe.

While the solar system consists of the Sun and the celestial bodies bound to it by gravity, and a galaxy, such as the Milky Way, is a massive system that contains millions or billions of stars along with their solar systems, they do not encompass everything that exists. The universe, on the other hand, contains all of these elements and more, making it the correct choice in this context.