Why is He2- unstable?

$\ce{He2-}$ according to MO theory should be possible with one electron in 2s sigma, but my professor said only at very low temperatures for some reason.

Why only at low temperatures? What is the relationship between the temperature and the stability of that bond? Why is this bond special in this way?

• There's no such molecule. Even argon matrix wouldn't help, even hard vacuum as it would probably have negative binding energy – Mithoron Feb 18 '16 at 16:34
• – Mithoron Feb 18 '16 at 16:40