been discovered that the universe contains much more matter than is accounted for by visible objects; stars, galaxies, nebulas and interstellar gas. This unseen matter is known as dark matter (dark means that there is a wide range of strong indirect evidence that it exists, but we have not yet detected it directly). The ΛCDM model is the most widely accepted model of the universe. It suggests that about of the mass and energy in the universe is dark energy which is responsible for the acceleration of the expansion of space, and about is dark matter. Ordinary ('baryonic') matter is therefore only of the physical universe. Stars, planets, and visible gas clouds only form about 6% of the ordinary matter. There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will ever be accessible. Some physicists have suggested various multiverse hypotheses, in which our universe might be one among many universes that likewise exist. Definition The physical universe is defined as all of space and time (collectively referred to as spacetime) and their contents. Such contents comprise all of energy in its various forms, including electromagnetic radiation and matter, and therefore planets, moons, stars, galaxies, and the contents of intergalactic space. The universe also includes the physical laws that influence energy and matter, such as conservation laws, classical mechanics, and relativity. The universe is often defined as "the totality of existence", or everything that exists, everything that has existed, and everything that will exist. In fact, some philosophers and scientists support the inclusion of ideas and abstract concepts—such as mathematics and logic—in the definition of the universe. The word universe may also refer to concepts such as the cosmos, the world, and nature. Etymology The word universe derives from the Old French word , which in turn derives from the Latin word . The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used. Synonyms A term for universe among the ancient Greek philosophers from Pythagoras onwards was () 'the all', defined as all matter and all space, and () 'all things', which did not necessarily include the void. Another synonym was () meaning 'the world, the cosmos'. Synonyms are also found