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The fraction of faint galaxies which exhibit optical variability should be a useful constraint on models of galaxy evolution. Other than supernovae, such variability implies a compact Seyfert- or QSO-like nucleus. We use this fraction to constrain flat cosmological models in which most of the dark matter is in compact objects in the $10^{5\pm 2} M_{\sun} $ range, producing variability via microlensing. We have conducted careful surveys for optical variability on deep CCD and photographic plates of blank fields. The faint galaxy CCD survey samples variability from 1984 -- 1994. With a threshold for detection of variability of $\Delta m \approx 0.07$ mag at $Bj = 22$ mag, to $\Delta m \approx 1$ mag at $Bj = 25$ mag, 0.2\% of the objects in a $\approx 10\arcmin$ field are variable. The photographic survey data is consistent: we detect 0.3$\pm$ 0.1\% of the objects to be variable over a $70\arcmin$ field, with a threshold ranging from $\Delta m \approx 0.35$ mag ($Bj \approx 20$ mag) to $\Delta m \approx 1$ mag at $Bj = 24.2$ mag.
Detection probabilities and error rates were checked by simulations and statistical analysis of fluctuations of sample sky spots. Various CCD systematic errors are discussed and analyzed. The fraction of objects that are found to be variable is small compared to other surveys. Our detection rate is about 20\% of the quasar fraction found via optical spectroscopy, near 22 Bj mag ($\approx 1.5\%$ of galaxies). It is also much smaller than the fraction of local galaxies that are Seyferts ($\approx 3\%$).
Finally, we have been able to discriminate AGNs from supernovae, finding a SN rate consistent with the local rate. As the SN rate is a good reflection of the rate of massive star formation, the lack of a dramatic increase in SN rate can also be used to constrain models of galaxy evolution.
