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We explore the dark matter distribution in ms1224.7+2007 using thegravitational distortion of the images of faint background galaxies. Projectedmass image reconstruction reveals a highly significant concentration coincidentwith the X-ray and optical location. The concentration is seen repeatably inreconstructions from independent subsamples and the azimuthally averagedtangential shear pattern is also clearly seen in the data. The projected masswithin a $2.76'$ radius aperture is $\simeq 3.5 \times 10^{14} h^{-1}M_{\sun}$. This is $\simeq 3$ times larger than that predicted if mass traceslight with $M/L = 275 h$ as derived from virial analysis. It is very hard toattribute the discrepancy to a statistical fluctuation, and a furtherindication of a significant difference between the mass and the light comesfrom a second mass concentration which is again seen in independent subsamplesbut which is not seen at all in the cluster light. We find a mass per galaxyvisible to $I = 22$ of $\simeq 8 \times 10^{12}h^{-1}M_{\sun}$ which, ifrepresentative of the universe, implies a density parameter $\Omega \sim 2$. Wefind a null detection of any net shear from large-scale structure with aprecision of 0.9\% per component. This is much smaller than the possibledetection in a recent comparable study, and the precision here is comparable toto the minimum level of rms shear fluctuations implied by observed large-scalestructure.Comment: 8 pages (no figures), latex, CITA/94/4, postscript file with figures  available by anonymous ftp in /cita/nick/ms1224/ at ftp.cita.utoronto.c

Dark matter in MS 1224 from distortion of background galaxies