We call the part of the universe we can see the observable universe. This means the light will never be able to reach us here on Earth, so we will never be able to see these parts of the universe. Some parts of the universe are expanding even faster than the speed of light. However, many astronomers think they must exist, because many astronomical observations would be hard to explain if they didn't. Just to be clear, dark matter and energy have not been observed directly (that is why they are called 'dark'). By studying the expansion of the Universe, astronomers have also realized most of the matter in the Universe may be in a form which cannot be observed by any scientific equipment we have. Because of this, astronomers call the mysterious energy causing the expansion dark energy. However, astronomers are still not sure what is causing the universe to expand. It is still expanding right now, and the expansion is getting faster. Since then, the universe has expanded to be at least 93 billion light years, or 8.80 ×10 26 metres, in diameter. Astronomers think that the Big Bang happened about 13.73 billion years ago, making the Universe 13.73 billion years old. There are many kinds of scientific evidence that support the Big Bang idea. The most used scientific model of the Universe is known as the Big Bang theory, which says the Universe expanded from a single point that held all the matter and energy of the Universe. The smallest, reddest galaxies, about 100, are some of the most distant galaxies to have been photographed. The equivalent area of sky that the picture occupies is shown in the lower left corner. It shows a variety of galaxies, each made of billions of stars.
High-resolution image of the Hubble ultra deep field. By measuring the redshift of galaxies, cosmologists soon discovered that the Universe is expanding (see: Hubble).
This started a new kind of astronomy called cosmology, in which astronomers study what these galaxies are made of and how they are spread out through so they can learn more about the history of the Universe and how it works. They also realized that other galaxies exist outside it, as far as we can see.
In the 1900s, even better telescopes led astronomers to realize that the Solar System is in a galaxy made of billions of stars, which we call the Milky Way. Meanwhile, Isaac Newton improved the ideas of gravity and dynamics ( equations) and showed how the Solar System worked.
The modern optical (uses visible light) telescope is still more advanced. By the middle of the 1800s, telescopes were good enough for other galaxies to be seen. The invention of the telescope in the Netherlands, 1608, was a very important moment in astronomy. This was very controversial at the time, and was fought by religious authorities, most famously by the Christian church (see Giordano Bruno and Galileo). Over hundreds of years, better observations led to Copernicus's Sun-centred model. The spheres circled about a round but unmoving Earth. They thought it had a set of celestial spheres which corresponded to the fixed stars, the Sun and various planets. Some ancient Greeks thought that the Universe has infinite space and has existed forever. Most early models had the Earth at the centre of the Universe. People have long had ideas to explain the Universe. Also see: Human timeline and Life timeline