Final answer:
The basic assumption that scientists hold about the universe is the cosmological principle, which states that the universe is the same everywhere on a large scale. This assumption is the starting point for theories that describe the entire universe.
Step-by-step explanation:
The basic assumption that scientists hold about the universe is the cosmological principle. This principle states that the universe is the same everywhere on a large scale, meaning it is isotropic and homogeneous. Scientists use this assumption as a starting point for nearly all theories that describe the entire universe. Without this assumption, it would be difficult to make any progress in studying the universe.