  Using Very Long Baseline Interferometry we have searched a sample of 300
compact radio sources for examples of multiple imaging produced by
gravitational lensing; no multiple images were found with separations in the
angular range 1.5--50 milliarcsec. This null result allows us to place a limit
on the cosmological abundance of intergalactic supermassive compact objects in
the mass range $\sim 10^{6}$ to $\sim 10^{8}$M$_{\odot}$; such objects cannot
make up more than $\sim 1%$ of the closure density (95% confidence). A
uniformly distributed population of supermassive black holes forming soon after
the Big Bang do not, therefore, contribute significantly to the dark matter
content of the Universe.
