Most likely Hydrogen. It is also the most flammable substance in the universe as well.

AIR. People forget AIR is an element

Hydrogen.

MOnkey would say helium maybe last time monkey suck air from a balloon made monkey squeak. maybe hydrogen not sure, wanna monkey around with that answer, now all this talk made me thirsty where's my beer...

Hydrogen, which is also the most abundant element in the known universe.

I think the lightest element is positronium. It's an electron and an positron that "orbit" each other. The two forms of positronium have very short mean lifetimes, though: 125ps and 140ns. This is way way lighter than hydrogen. Some people might not consider this "exotic atom" a real element, though. Those people would consider Hydrogen to be the right answer, which is the most abundant element in the Universe.

Entropy. It's a 0 on the periodic charts. It's composed of nothing.

oxygen--O.

When i had chemistry in school, it was hydrogen back then.

As far as we now, it would be hydrogen: "Hydrogen is a chemical element. It is the simplest atom in the Universe. On a periodic table of the elements, hydrogen is on the top row, usually either on the left or on its own above the transition metals." "It is: A gas at room temperature A non-metal The lightest of all the elements in the periodic table The lightest element in the Universe The most common element in the Universe " Source and further information: http://simple.wikipedia.org/wiki/Hydrogen 2) However, there would also be another possibility: "Exotic matter is a hypothetical concept of particle physics. It covers any material which violates one or more classical conditions or is not made of known baryonic particles. Such materials would possess qualities like negative mass or being repelled rather than attracted by gravity. It is used in certain speculative theories, such as on the construction of wormholes. The closest known real representative of exotic matter is a region of pseudo-negative pressure density produced by the Casimir effect." Source and further information: http://en.wikipedia.org/wiki/Negative_mass#Negative_mass

Terpsichorian, a hyperbolic allotrope of hydrogen, is in fact two and a third times lighter than hydrogen. The Zeppellin company were planning a terpsichorian filled airship - the Zeppelin Mk 307, when the war abruptly ended. Unfortunately, terp (O God, don't make me spell out it again) is also two and a third times more explosive than hydrogen. The Zeppelin Mk 307 was therefore planned to double as a terrible weapon of aerial destruction or a lunar landing craft.

Hydrogen is what we know now as the lightest element in the Universe... all light elements are unstable ,as all heavy elements are like uranium.. Tha balance lies in Iron a very stable element..The Universe itself shows this .. But what created hydrogen..I predict we will find another element if not more,lighter than hydrogen.. I also predict we will find heavier elements..Theroized in the hearts of black holes... and when a trillion black holes join together ..it just might be the creation of a new universe..thereby creating a cycle.. The key to all this is finding both ends or realizing them..(theroy)... We are just a speck on a speck on a speck x100... we see the Universe in slow motion because of this...,but if you were the oposite you would see the exposion as a firecracker going off...If we think smaller than an atom as an atom would think in its world time would practically stop and the big bang is just about to pop.. We would understand what that lighter element was and would have a greater understanding of where it might have come from..thereby taking us to the heaviest... Its all a cycle and when all ends it will be recreated once again and may be already happening somewhere else infinatly beyond what we see or what we can imagine..Until we imagine the unimaginable..

Hydrogen. It is only made from 1 proton, 0 neutron's, and 1 electron. The only thing which would be ever-so-slightly lighter would be the hydrogen ion H+ which is just made from 1 proton only (although this is in ion, not an element).

7-2-2017 We seem to have a bunch of people pretending to know things when they don't. You just have to watch out for such people. There are lots of them. "Element" is a carefully defined word and there exists a list of all that we know about. It is called "Periodic Table Of The Elements" and you can buy one in any college book store. Hydrogen is the lightest, there are 91 naturally occurring elements (#43 is not natural), and no particular limit has been found yet for elements made in a laboratory, except that the bigger they get the harder it is to find the next one.