Simply, space is everything in the universe beyond the top of the Earth’s atmosphere – the Moon, where the GPS satellites orbit, Mars, other stars, the Milky Way, black holes, and distant quasars. Space also means what’s between planets, moons, stars, etc – it’s the near-vacuum otherwise known as the interplanetary medium, the interstellar medium, the inter-galactic medium, the intra-cluster medium, etc; in other words, it’s very low density gas or plasma.
Another simple, but profound, answer to the question “What is space?” is “that which can be measured with a ruler”. And why is this a profound answer? Because thinking about it lead Einstein to develop the first the theory of special relativity, and then the theory of general relativity. And those theories overthrew an idea that was built into physics since before the time of Newton (and built into philosophy too); namely, the idea of absolute space (and time). It turns out that space isn’t something absolute, something you could, in principle, measure with lots of rulers (and lots of time), and which everyone else who did the same thing would agree with you on.
Space, in the best theory of physics on this topic we have today – Einstein’s theory of general relativity (GR) – is a component of space-time, which can be described very well using the math in GR, but which is difficult to envision with our naïve intuitions. In other words, “What is space?” is a question I can’t really answer, in the short space I have in this Guide to Space article.
One amazing fact about the space is that in space, no one can hear you scream. This is because there is no air in space – it is vacuum. Sound waves cannot travel through vacuum.
'Outer space' begins about 100 km above the Earth, where the shell of air around our planet disappears. With no air to scatter sunlight and produce a blue sky, space appears as a black blanket dotted with stars.
Space is usually regarded as being completely empty which is truly a misconception. The vast gaps between the stars and planets are filled with huge amounts of thinly spread gas and dust. Even the emptiest parts of space contains at least a few hundred atoms or molecules per cubic meter.
Space is also filled with many forms of radiation that are dangerous to astronauts. Much of these are infrared and ultraviolet radiations that come from the Sun. High energy X-rays, gamma rays and cosmic rays – particles travelling close to the speed of light – arrive from distant star systems.