We cross-correlate QSOs from the 2dF QSO Redshift Survey with groups of galaxies .
In the southern region of the 2dF we utilise galaxies from the APM Survey .
In the northern strip , galaxies are taken from the recent Sloan Digital Sky Survey Early Data Release .
Both galaxy samples are limited to a depth B < 20.5 .
We use an objective clustering algorithm to detect groups in these galaxy catalogues .

A 3 \sigma anti-correlation is observed between 2dF QSOs and galaxy groups , confirming the effect found by Boyle , Fong & Shanks in an independent dataset .
This paucity of faint QSOs around groups can not be readily attributed to a selection effect and is not due to restrictions on the placement of 2dF fibres .
By observing the colours of QSOs on the scales of the anti-correlation , we limit the influence intervening dust in galaxy groups can have on background QSO flux , finding a maximum reddening on the scale of the anti-correlation of E ( b _ { j } - r ) \leq 0.012 at the 95 per cent level .
The small amount of dust inferred from the QSO colours would be insufficient to account for the anti-correlation , supporting the suggestion by Croom & Shanks that the signal is likely caused by weak gravitational lensing .
The possibility remains that tailored dust models involving grey dust , heavy patches of dust , or a combination of dust and lensing , could explain the anti-correlation .

Under the assumption that the signal is caused by lensing rather than dust , we measure the average velocity dispersion of a Singular Isothermal Sphere that would cause the anti-correlation , finding \sigma \sim 1150 { ~ { } km~ { } s ^ { -1 } } .
Simple simulations reject \sigma \sim 600 { ~ { } km~ { } s ^ { -1 } } at the 5 per cent significance level .
We also suppose the foreground mass distribution consists of dark matter haloes with an NFW profile and measure the typical mass within 1.5 ~ { } h ^ { -1 } ~ { } { Mpc } of the halo centre as M _ { 1.5 } = 1.2 \pm { 0.9 } \times 10 ^ { 15 } ~ { } h ^ { -1 } ~ { } { M } _ { \odot } .

Regardless of whether we utilise a Singular Isothermal Sphere or NFW dark matter profile , our simple lensing model favours more mass in groups of galaxies than accounted for in a universe with density parameter \Omega _ { m } = 0.3 .
Detailed simulations and galaxy group redshift information will significantly reduce the current systematic uncertainties in these \Omega _ { m } estimates .
Reducing the remaining statistical uncertainty in this result will require larger QSO and galaxy group surveys .