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The trajectory of the center of light of an unresolved binary is differentfrom that of its center of mass. Binary-induced stellar centroid wobbling cantherefore be detected as an excess in the goodness-of-fit of the single-starastrometric model. We use reduced $\chi^2$ of the astrometric fit in the {\itGaia} Early Data Release 3 to detect the likely unresolved double white dwarfs(DWDs). Using parallax-based distances we convert the excess of reduced$\chi^2$ into the amplitude of the centroid wobble $\delta a$, which isproportional to the binary separation $a$. The measured $\delta a$ distributiondrops towards larger wobble amplitudes and shows a break around $\delta a\approx 0.2$ where it steepens. The integral of the distribution yields DWDfraction of $6.5 \pm 3.7$ per cent in the range $0.01 < a (\text{au}) < 2$.Using synthetic models of the Galactic DWDs we demonstrate that the break inthe $\delta a$ distribution corresponds to one side of a deep gap in the DWDseparation distribution at around $a\approx 1$ au. Model DWDs with separationsless than several au shrink dramatically due to (al least one) common envelopephase, reshaping the original separation distribution, clearing a gap andcreating a pile-up of systems with $a\approx 0.01$ au and $\delta a < 0.01$.Our models reproduce the overall shape of the observed $\delta a$ distributionand its normalisation, however the predicted drop in the numbers of DWDs beyondthe break is steeper than in the data.

A gap in the double white dwarf separation distribution caused by the common-envelope evolution: astrometric evidence from Gaia