  A deep narrow-band survey for Ly-alpha emission carried out on the VLT-FORS2
has revealed 98 Ly-alpha candidates down to a flux limit of 4.e-18 erg/s/cm^2
in a volume of 5500 comoving Mpc^3 at z=2.4 centered on the hyperluminous
quasar HE0109-3518. The properties of the detected sources in terms of their i)
equivalent width distribution, ii) luminosity function, and iii) the average
luminosity versus projected distance from the quasar, all suggest that a large
fraction of these objects have been fluorescently "illuminated" by HE0109-3518.
This conclusion is supported by comparison with detailed radiative transfer
simulations of the effects of the quasar illumination. 18 objects have a
rest-frame Equivalent Width (EW0) larger than 240A, the expected limit for
Ly-alpha emission powered by Population II star formation and 12 sources among
these do not have any continuum counterpart in a deep V-band imaging of the
same field. For these, a stacking analysis indicates EW0>800A, effectively
ruling out Ly-alpha powered by internal star formation. These sources are thus
the best candidates so far for proto-galactic clouds or "dark" galaxies at
high-redshift, whose existence has recently been suggested by several
theoretical studies. Assuming they are mostly ionized by the quasar radiation,
we estimate that their gas masses would be about 10^9 Msun implying that their
star formation efficiencies (SFE) are less than 1.e-11 yr^-1 several times
below the SFE of the most gas-rich dwarf galaxies locally, and five hundred
times lower than typical massive star-forming galaxies at z~2. We have also
discovered extended, filamentary gas, also likely illuminated by the quasar,
around some of the brightest continuum-detected sources with EW0>240A. This
emission is compatible with the expectations for circum-galactic cold streams
but other origins, including tidal stripping, are also possible.
