According to the Standard Big Bang Model, the universe as
we know it came into existence about 15 billion years ago.
Since then, it has expanded and cooled. As its properties
have changed over time, its consituents (from elementary
particles to atoms, stars and galaxies) have likewise changed.
The very earliest times include the era when the forces were
unified according to the Grand Unified Theories, or GUTs, one
of which is the theory of Inflation.