The Big Bang Theory, in its simplest form, can be condensed into four basic steps. First, the universe formed because of a violent cosmic bomb. All of the concentrated matter of the universe expanded outwards in the form of a very hot gas to create both space and time. Then, this hot gas condensed to form galaxies and stars. Galaxies eventually separated and are still moving away from us - and each other - today.
This is much like a chocolate chip cookie. When you place the cookie dough into the oven, the chocolate chips are close together. Once cooked, these chocolate pieces are further apart. The cookie (like space) grows and expands as a whole and although they melt, the chocolate pieces (like galaxies that are held together by gravity) do not grow in size. Instead, these pieces become further apart from each other. Evidence has shown that the rate at which these galaxies are moving away from each other is increasing.
The source of the universe’s accelerating growth is not currently known. To explain it, scientists have proposed the existence of special “dark” forces. Current cosmology models suggest that “dark matter” and “dark energy” make up most of the universe (over 95%). This leaves only a mere 5% for ordinary matter like planets, stars and all the gas that exists between them.
Dark matter is a term and concept used to explain why the universe holds as much mass, or “stuff” as it does. When astronomers first discovered that the largest galaxies had a much greater mass than the stars, dust and gas that they contained, they needed a way to explain this missing mass. Even though dark matter makes up such a significant portion of the universe, we can’t see it. This is because, to our knowledge, dark matter does not emit or reflect any form of radiation (including light or x-rays) so we can only recognize that it exists by how it interacts with what we can see.
Dark matter’s even more mysterious companion is dark energy. This term is given to the force responsible for making the universe expand at an increasing rate. Distant galaxies are moving away from us and this increasing speed of our universe’s growth is not well understood by scientists. Einstein also proposed a “cosmological constant” to explain this strange force that acts against gravity, but he abandoned the idea when Edwin Hubble discovered in 1929 that galaxies are moving away from each other. Despite Einstein regarding this constant as a big “blunder”, scientists have since supported it because dark energy and this constant support the discovery in the late 1990's that the rate at which the universe is growing is in fact accelerating.
To better understand these “dark” concepts, scientists are trying to investigate what dark matter could be made of. For example, Switzerland’s Large Hadron Collider at CERN, the European Organization for Nuclear Research, is attempting to create dark matter in laboratory conditions. This research is investigating if dark matter is made up of special particles unlike those known to us in standard physics. Other efforts are sending spacecraft into space to investigate the role of dark matter and energy in our universe's expansion. For example, the Hubble Space Telescope has been observing the universe since 1990 and has helped verify the universe’s rate of expansion.
An upcoming mission that hopes to better explain dark matter and dark energy is the European Space Agency's Euclid mission, which is due for launch in 2022. The spacecraft will feature a 1.2 meter-diameter telescope to collect data. An estimated 2 billion galaxies are intended to be mapped by this mission over 6 years and it will collect information in multiple ways. One of these is the “gravitational lensing” technique. When imaging large-scale structures in our universe, light rays coming towards us from distant galaxies pass through the strong gravitational field of dark matter. Because of this, it is refocused, bent, and distorted by this lensing concept. In the image, distant and lensed galaxies can look stretched and pulled as the light passes closer to the closer galaxy clusters in the image (see below). In other words, mass bends light. Another method that will be used by the Euclid mission is called “baryon acoustic oscillation”, which measures the changes in density of our universe’s normal matter. Together, these techniques will measure the distances to many galaxies and will map the formation of large-scale structures in the universe. This mission will help us better understand the evolution and development of dark energy in our universe. Such knowledge is important because this mysterious force was once just a tiny presence following the Big Bang but it has since grown to take up the majority of our cosmos, hence why scientists are eager to understand dark matter and energy.
Strong Gravitational Lensing as Observed by the NASA/ESA Hubble Space Telescope.