The universe refers to all of space, time, matter, and energy that exists, including galaxies, stars, planets, and all other forms of matter and energy. It encompasses everything that we can observe, as well as the vast expanse beyond our direct observations.
The age of the universe is estimated through various scientific methods, primarily based on measurements of the cosmic microwave background radiation (CMB) and the observed expansion of the universe. According to the most widely accepted cosmological model, the Big Bang theory, the universe originated from an extremely hot and dense state approximately 13.8 billion years ago.
The age of 13.8 billion years is obtained by measuring the properties of the cosmic microwave background radiation, which is the remnant heat radiation from the early stages of the universe. Precise measurements from satellite missions, such as the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite, have provided highly accurate data that support this age estimation.
It's important to note that the concept of "age" in this context refers to the time since the Big Bang and the emergence of the universe as we know it. The origins of the universe before the Big Bang, including questions about the nature of time itself, remain active areas of scientific research and are subjects of ongoing investigation and theoretical exploration.