Can Unclustered Matter Close the Universe?

Any matter unclustered on the supercluster scale must be "superhot" and became nonrelativistic only very recently. Such an unclustered collisionless background would dominate the radiation universe at nucleosynthesis and would later dampen the growth by gravitational instability of structure in the clustered component. If structure evolved by gravitational instability, the universe cannot be dominated by unclustered, collisionless, nonrelativistic dark matter, i.e., Ω₀~Ωcl (observed for the clustered component). If indeed Ωcl=0.35±0.15 then (barring a fine-tuned cosmological constant) the universe must be open, rather than flat.