For the ancient Greeks, the order, beauty, harmony and perfection in their perceptions about nature were of great importance. Cosmos is a Greek word meaning “order”, but also “harmony” and “beauty”. When speaking about the Cosmos, the ancient Greeks meant the Universe, viewed as perfect, beautiful, ordered and harmoniously organised system. The perfection of the Cosmos was opposed by the Chaos.

At present, in “Cosmos” we include the whole material world outside the Earth and its atmosphere. Often, Cosmos means the Universe as such, viewed as something unified and obeying common laws. Universe, however, is a much wider concept, filled with philosophical sense as well and spreading outside astronomy as science – in fact Universe is the whole world that surrounds us. We call Universe everything that exists and that we already know, as well as that, that we will discover in the future.

For the astronomers, the Universe is that part of the world which is accessible (now and in the future) for exploration with scientific methods. The astronomical Universe describes the whole Cosmos, which in turn includes all known and unknown celestial objects, as well as the space between them, and which is observable now or will be in the near future.

Our understanding about the Universe has changed significantly in the past few years. Up until a century ago, widely accepted was the idea of a static, unchanging Universe, without beginning and end, which has always existed and will continue to exist forever. At present, the recent discoveries and observed facts established the concept that the Universe is a constantly evolving dynamic structure with all its variety of material and energetic forms. It has not existed forever and is developing in time. What is more – the Universe in which we live probably is not the only one but is a part of something bigger, that we call Multiverse. According to the modern understanding, about 14 billion years ago, a super massive explosion called Big Bang has given the beginning of the observable Universe and the laws of physics that govern it. Since then, the Universe is expanding and will probably continue to expand forever. This expansion, as well as the very weak electromagnetic echo from the Big Bang, are observable even at the moment. So it is not by chance that we talk about “observable Universe” or about “observable matter” - in fact the most part of the total mass of the Universe can not be observed directly even with the most advanced instruments. This part of the Universe we call dark matter – the term comes from the fact that it is not possible to observe it directly. There is not even a clear concept of what kind of matter dark matter actually is.

Besides the dark matter, there is huge amount of dark energy hidden in the Universe. It is an energy of the cosmic vacuum, which is not connected to any kind of matter and manifests as anti-gravity – a force that opposes gravity. Almost 3⁄4 of the Universe consists of dark energy. Dark energy is what makes the Universe to accelerate its expansion. It is also a property of space. The rest of the Universe consists mostly of dark matter, whose composition is still unclear. Only a very small part comprises the “ordinary”, visible matter from which planets, stars and galaxies are made. So, in total, the Universe contains only 4% observable ordinary matter(atoms and particles), 23% dark matter, and whole 73% of the mysterious dark energy.

While gravity acts upon all forms of matter, the “anti-gravity” of dark matter manifests only on very large scales. It forces the Universe to expand with an acceleration – such an expansion would not be possible if the gravity was the only dominating force. The traces of the anti- gravity’s effects were observed indirectly in the light from supernovae, situated at enormous distances from us and the discovery of these traces led to the Nobel price for Physics in 2011.