What is the Cosmic Microwave Background
The Cosmic Microwave Background, or CMB, is leftover radiation from the Big Bang explosion that created the universe. It is like the afterglow of that massive explosion, and we can still detect it today using special instruments in space. By studying this ancient light with telescopes, scientists can figure out how long ago the universe started.
How Scientists Measure the Universe's Age
Scientists use multiple methods to determine the universe's age. First, they measure the Cosmic Microwave Background radiation patterns using space telescopes like the Planck satellite. Second, they observe how fast distant galaxies are moving away from us, which tells us how long it has taken for them to reach their current distance. Finally, they study the brightness and age of the oldest stars. When all these methods are combined, they consistently show the universe is 13.8 billion years old.
Why We Can Trust This Measurement
Multiple independent methods all produce the same answer, which makes scientists confident in the age estimate. Observations from different telescopes, made over many years, have confirmed this measurement. Scientists have also checked their work by comparing the age of the oldest known objects in the universe with the calculated age, and they match up correctly.
What Happened at the Beginning
According to the Big Bang theory, the universe started as an extremely hot, dense point and has been expanding ever since. The light we detect as the Cosmic Microwave Background was released about 380,000 years after the Big Bang began. By measuring this ancient light and understanding how the universe has expanded since then, scientists calculated that the Big Bang happened 13.8 billion years ago. 
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