Which event led to the formation and expansion of the universe?

The event that led to the formation and expansion of the universe is the Big Bang. This theory posits that the universe began as an incredibly hot and dense point approximately 13.8 billion years ago. From this initial singularity, it rapidly expanded and cooled, leading to the formation of fundamental particles and eventually atoms, stars, galaxies, and all other structures in the universe.

The Big Bang is supported by several key pieces of evidence, including the observable expansion of the universe, the cosmic microwave background radiation (the remnant heat from the initial explosion), and the abundance of light elements that formed during the first few minutes after the event. This foundational theory provides the framework for understanding the evolution of the cosmos and is widely accepted within the scientific community.

Other options, while relevant in the context of cosmology, do not directly explain the initial event that set everything in motion. The solar nebula refers to the formation of our solar system specifically, dark matter is a component of the universe that influences its structure and evolution but does not explain the origin, and Hubble's law describes the rate of expansion of the universe, which is a consequence of the initial event rather than the event itself.