  Decaying dark matter (DDM) can be tested via different astrophysical and
cosmological probes. In particular, particles in the $\sim$ 9.5 - 30 eV mass
range that decay into monochromatic photons, would contribute to the
extragalactic background light (EBL) in the ultraviolet (UV) bandwidth. In this
work, we show that an intriguing improvement to the constraints on such DDM
models can come from broadband UV surveys, such as GALEX or the upcoming
ULTRASAT satellite. These provide diffuse light maps of the UV EBL, integrated
over a wide redshift range. The cross correlation between intensity
fluctuations in these maps with a reference spectroscopic galaxy survey, can be
used to reconstruct the redshift evolution of the EBL intensity; in this way,
it is also possible to detect signatures of contributions from DDM. We forecast
the constraining power of (GALEX+ULTRASAT)$\times$DESI, and we show they will
be able to detect DDM with decay rate up to $\mathcal{O}(10^{-26}\,{\rm s})$.
In the context of axion-like particles (ALP), our forecasts can be converted to
constraints on the ALP-photon coupling; our results show this technique will
test ALP with coupling $\lesssim\mathcal{O}(10^{-12}\,{\rm GeV^{-1}})$, more
than an order of magnitude better than current bounds in this mass range.
