Final answer:
The idea that the cosmos is expanding originated from the Big Bang theory, which suggests that the universe began with an explosive event. This idea is supported by observational evidence.
Step-by-step explanation:
The idea that the cosmos is expanding is believed to have originated from the Big Bang theory. The Big Bang theory suggests that the universe began with an explosive event, around 13.7 billion years ago, which caused the expansion of space itself. This idea is supported by observational evidence, such as the redshift of distant starlight, which indicates that galaxies are moving away from us.