Final answer:
The universe is approximately 13.8 billion years old according to the big bang model, which is supported by observations like cosmic background radiation and the formation of hydrogen atoms 380,000 years after the Big Bang.
Step-by-step explanation:
According to the big bang model, the universe began from a singularity approximately 13.8 billion years ago. This event marked the start of space-time and has since been evidenced by various observations such as the isotropic expansion of the universe and the cosmic background radiation.
One of the significant milestones postulated by the Big Bang theory is the moment, roughly 380,000 years after the event, when the universe cooled sufficiently for protons and electrons to combine and form hydrogen atoms, resulting in a universe that became transparent to light and setting the stage for the formation of the first stars and galaxies during the subsequent hundreds of millions of years.