# Does every object in the Universe have gravity? Space has no gravity, why?

My science teacher told me that every object in Universe has gravity but space is a part of Universe too, so, why does the space have no gravity?

• Are you saying that "space" is an "object"? I don't think that's what your teacher meant. – Steve Linton Mar 10 '18 at 19:28
• When you say "space has no gravity" do you mean that empty space doesn't pull things towards it (which is more or less true) or are you referring to the way astronauts (and other things) float around in space and are not pulled towards the floor of the ISS or whatever. That's because the floor and the astronaut are being pulled equally be Earth's gravity and constantly falling around the Earth. There is a gravitational force between the astronaut and the ISS (or anything else in it) but it is too weak to notice without sensitive instruments. Can you clarify? – Steve Linton Mar 10 '18 at 19:32

A quick search up on google you would find this:

Gravity causes every object to pull every other object toward it. Some people think that there is no gravity in space. In fact, a small amount of gravity can be found everywhere in space. Gravity is what holds the moon in orbit around Earth.

Why is this? That's because:

Outer space is the closest known approximation to a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along their ideal orbits. However, even the deep vacuum of intergalactic space is not devoid of matter, as it contains a few hydrogen atoms per cubic meter.

If there's matter in space, thus this makes this statement valid.

Yes, everything that has mass will cause a gravitational pull on other objects.

• This isn't really an answer. The OP states that the universe has "objects" and "space" and wants to know if the "space" has gravity. Your answer boils down to "there are objects throughout space, and objects have gravity". – JBentley Feb 5 '16 at 15:59

In the intergalactic medium — the most dilute regions of the Universe between the galaxies — as CipherBot writes you'll find roughly one hydrogen per cubic meter, i.e. the density is $\sim10^{-6}\,\mathrm{atoms}\,\mathrm{cm}^{-3}$, or $\sim10^{-30}\,\mathrm{g}\,\mathrm{cm}^{-3}$, or, in terms of energy (since mass and energy are equivalent through $E=mc^2$), $\sim10^{-9}\,\mathrm{erg}^\dagger$. In addition to this, you'll find 5–6 times as much dark matter.

Space itself can't really be considered "an object". Nevertheless, even ignoring the normal and dark matter, space does have energy: the so-called dark energy. We don't really know much about it, but we can measure its presence through its effect on the expansion of the Universe. But whereas normal and dark matter decelerates this expansion, dark energy has the opposite effect of accelerating the expansion. And since the energy density of dark energy is more than twice that of the other two components together, it actually dominates the dynamics of the Universe.

So, in this regard you can say that space does have gravity, although it's a "negative gravity". This phrasing is a bit misinterpreted, though. The dark energy is thought to be a negative pressure. Pressure has a energy density associated with is, so negative pressure has a negative energy density.

$^\dagger$$1\,\mathrm{erg}\equiv10^{-7}\,\mathrm{Joule}$.