The Big Bang model describes the early Universe as an extremely hot, dense state about 13.8 billion years ago. Since then, space has expanded and cooled; it was not an ordinary explosion into pre-existing empty space.
This picture is supported by measurements of cosmic expansion and the cosmic microwave background, ancient radiation left from the early hot era. These observations constrain the model, even though they do not answer every question about the Universe.
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How did stars and galaxies form?
After the Universe cooled, neutral atoms could form. Gravity gradually gathered gas into denser regions, where the first stars were born and galaxies began to grow.
Galaxies continued to form stars, interact, and merge over cosmic time. Our Sun is one star in the Milky Way, and Earth is part of the Solar System rather than the centre of the galaxy or the Universe.
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Why do telescopes look into the past?
Light travels at a finite speed, so a distant object is seen as it was when its light began its journey. Looking farther away therefore means looking further back in cosmic time.
Webb observes infrared light, including light from distant galaxies stretched by the expansion of space. It helps scientists study early galaxies, but it does not photograph the moment of the Big Bang itself.