The latest experimental evidence suggests that the universe is made up of just 4% ordinary matter, 23% cold dark matter and 73% dark energy. These values were obtained by fitting data from measurements of the Cosmic Microwave Background radiation to models of our universe. Dark Energy is still a great mystery to scientists. One possibility for this additional energy is “quintessence”, a fifth fundamental force to go with gravity, the two nuclear forces and the electromagnetic force. This fifth force could be responsible for accelerating the rate of expansion of the Universe. Einstein was the first to introduce such a force when he added a term called the cosmological constant,  to his theory of general relativity. He later removed it, calling it his “greatest mistake”. Recently, it has been re- introduced by scientists trying to explain their observations of the Universe. Ordinary Matter is what stars, planets and people are made from. All matter is made up of combinations of 6 quarks, 6 leptons and their antiparticles. Particles such as the electron and neutrino are known as leptons. Particles such as the proton and neutron are made of three quarks, and are called baryons. Scientists often refer to ordinary matter as baryonic matter. + - Dark matter is matter we cannot see directly when we observe the Universe. It could be made up of one of many different things: cold baryonic matter that cannot be seen by our telescopes, exotic particles called WIMPS which react only weakly with matter, and so are so are difficult to detect, or super-massive black holes at the centres of distant galaxies….. The exact amount of dark matter could determine the ultimate fate of our Universe - will it collapse or will it expand forever? Dark Matter Candidates Dark Matter Detectors Background object Massive compact object Observer When light passes by a massive object its path is bent. If a massive object lies between us and a distant source, the light rays from the source may be focused changing the appearance of the source. This effect is known as gravitational lensing. When the massive object is something quite small, such as a brown dwarf, the effect is known as gravitational micro-lensing. By observing many stars over a period of time to see how their appearance changes, scientists hope to detect the presence of massive objects lying between the Earth and the stars – objects that could not normally be detected. Instruments such as the CDMS (Cryogenic Dark Matter Search) are being used to search for WIMPS. The detectors have to be carefully shielded from other sources of radiation, and are often built at the bottom of deep mines. Super-Kamiokande detects radiation from neutrinos that move faster than the speed of light, using an effect analogous to the sonic boom from fast aircraft MACHOS Massive Compact Halo Objects WIMPS Weakly Interacting Massive Particles Neutrinos A Brown Dwarf. A ball of gas too small to ignite fusion reactions and become a star. The spot at the image centre is believed to be a brown dwarf. White Dwarf stars. The remains of dead low mass stars are circled in this image made with the Hubble Space Telescope. Neutron Star. The remains of a massive, dead star. This image is of the Crab Pulsar a rapidly rotating neutron star. A Black Hole can be formed when massive stars collapse. Super- massive black holes, millions of times more massive, are believed to reside in most galaxies. A WIMP is a particle many times more massive than a proton that only interacts weakly so is very hard to detect. They could make up as much as 99% of dark matter. Neutrinos are like electrons, however they have no charge. Long believed to be mass-less, recent experiments have suggested that they may in fact have a tiny mass.