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When light from a distant galaxy travels toward us, dark matter along the way bends the rays slightly, like imperfect glass. Astronomers measure these distortions to understand where the invisible mass lurks. But galaxies also have their own, "intrinsic" distortions: they align not randomly but along the invisible threads of the cosmic web. It's like trying to gauge the curvature of a window pane when the panes themselves are slightly warped to begin with.

Scientists from the Dark Energy Survey collaboration tested a "self-calibration" method designed to separate intrinsic distortions from those caused by dark matter. They applied it to real telescope data from the first year of observations. For distant galaxies, a signal emerged, but for nearby ones, the results were ambiguous. The likely culprit: errors in estimating galaxy distances, because the self-calibration method is highly demanding on the accuracy of those estimates.

The authors showed how different ways of reconstructing the distance distribution affect the outcome. The same set of galaxies can yield either a faint hint of a signal or none at all. This is an important step for future sky surveys that will gather hundreds of times more data.