The scale of the universe is uncomprehensible.
As far as men can 'see' the universe (by means of various techniques), the size of the universe is estimated to 1.6 billion million (so 16 with 23 zeros) kilometers.
The real size of the universe is most probably much larger, actually it is already of an almost infinite size and still expanding.
Also the universe is not a firm 'something', however a physical action of mass, dimension and time.
Image of a part of the universe we can see, taken by the satellite ROSAT in the middle the Milkyway.
The universe is alive.
The universe is 'everything', it contains all matter, energy, space and even time that exists. Matter is present as solitairy atoms, whafts of gases or tiny dustparticles, but also as very large objects like planets, stars, Black Holes, dense gas and dustclouds or even dark matter.
The larger part of the ordinary matter in the universe is found in the galaxies as large objects or as interstellar matter. About 1% of all interstellar matter in the universe is tiny interstellar grains, which are believed to be frozen water and carbon dioxide. The rest of it is gas, mostly hydrogen and helium.
The universe is very dynamic, and constantly changing.
Since the Big Bang it is still expanding, and the galaxies are drifting further away from each other. Hubble's law states that the greater the distance between two galaxies, the greater their relative speed of separation, this means that the expansion of the universe is roughly uniform. The galaxies itself are constantly changing too, not only as stars forming and dying, but also as result of interaction between the galaxies in clusters. Meanwhile energy is generated, transformed and destructed and even time is delayed or accelerated.
The universe is also a mixture of gravitational and electromagnetic forces as well as electromagnetic radiation of all natures like light, röntgen-, radioradiation, and other types of energy.
Although the universe contains 'everything', large parts of it appear to be absolutely empty. All matter, dark matter included, that is found in the universe is concentrated in galaxies, that are grouped in clusters and sometimes superclusters. The clusters are very irregulary distributed over the universe and occupy only a small part of all space, the larger part is void.
The stars and galaxies are separated by inimaginable empty space. If the Sun was as large as a pea, the Earth would be a tiny dustparticle at about 75 cm away from it. Keeping in mind the same idea the nearest stars would be situated at a distance of more than two hunderd kilometers away. In between we would only find emptyness, or maybe some separated atoms or miscroscopically small dustparticles here and there.
Also in between the galaxies and clusters such enormous empty spaces exist, in these empty spaces you could travel during tens of millions of years without even coming near a galaxy. Our Milkyway is part of a complex of superclusters. We are situated at the outeredge of this supercluster, at our side there is a large gap of more than one hundred lightyears of a large void.
A conclusion of the General Theory of Relativity is that distances in space are influenced by the matter in the space. Due to mass the space is curved, these curves increase the distances between objects and so the traveling time between them is prolonged.
The greater the mass the greater the curving, the longer the traveling time will be.
Albert Einstein explained that the universe is not a three dimensional space with a variable length, width and height. Space is four dimensional, time, which is a variable too, is the fourth dimension. Therefore he called it spacetime.