The virtual particle pairs that spontaneously form and disappear have an affect on the universe, but its not gravity and as such, probably cannot constitute the 'dark matter' that would be necessary to recollapse the universe. These particles do have an affect on the universe, however small. One effect (still theoretical, as there is no practical way to measure it) is through black hole evaporation. Another example (observable, predictable and measurable) is the Casimir Effect.
Personally, I'm betting on dark matter (should our universe be a closed system that will eventually recollapse or slow its expansion unto heat death) will be found in black holes, or possibly exotic (not in terms of energy) forms of matter that exist not in our 3+1 space, but in the rolled up dimensions predicted by string theory.
Remember, also, that Einstein introduced a cosmological constant to force the universe to be static (not to make it expand to match Hubble), as he felt that expansion was right out. It wasn't until much later that Hubbles' observations were explained sufficiently well that the cosmological constant had to be thrown out.
Zero point energy, for those who are curious, is the energy of a system at 0K. To quote Gribbin: "This minimum energy cannot be precisely zero because of quantum uncertainty. In Quantum Field Thoery, the lowest energy state of a field (its ground state) is also non-zero, for the same reason, giving the quantum vacuum a complex structure, which can be probed experimentally." One example of this complex structure is the aforementioned Casimir effect.
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--Mik
http://www.404error.comruhkferret on ICQ/AIM
"Your guy was a little SQUARE! You had to use your IMAGINATION! There were no multiple levels or screens. There was just one screen forever and you could never win the game. It just kept getting harder and faster until you died. JUST LIKE LIFE." --Ernie Cline
