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Water is a very common substance, not just on Earth, but also in space. Although water usually occurs in space as a solid and occasionally as a gas and relatively rarely as a liquid, the most common compound is H₂. I believe based on the abundance of elements in the universe (ignoring helium because it is a noble gas) that water is the most common multielemental (having more than one type of element in its structure) compound. Are there more common ones, or is water the most common one?

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    $\begingroup$ Because space is so empty, there's a fair chance the most common compound has only two atoms. Things like $\ce{HO}$ and $\ce{CH}$ can last for a very long time if there's nothing else to react with. $\endgroup$ – Nicolau Saker Neto Jun 6 at 22:05
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    $\begingroup$ Ignoring He isn't good idea. HeH+ cation is one of most common in the Universe. $\endgroup$ – Mithoron Jun 6 at 22:10
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    $\begingroup$ Helium is not only a noble gas, but also the least reactive one. Mithoron might be right, but I am doubtful though I think it is a possibility. $\endgroup$ – Number File Jun 6 at 22:23
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    $\begingroup$ Well. if you ask only about neutral molecules then OK, but $\ce{HeH+}$ is, along with $\ce{H3^+}$, very common in space, so I think you should decide whether the question should be less or more broad. $\endgroup$ – Mithoron Jun 6 at 22:37
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    $\begingroup$ Related, though I can't make any guarantees about the source: reddit.com/r/askscience/comments/3mr1i7/… $\endgroup$ – Tyberius Jun 6 at 22:52

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