  Distinctive large-scale structures have been identified in the spatial
distribution of optical galaxies up to redshift z ~ 1. In the more distant
universe, the relationship between the dust-obscured population of star-forming
galaxies observed at millimetre wavelengths and the network of cosmic filaments
of dark matter apparent in all cosmological hydrodynamical simulations is still
under study. Using the NIKA2 dual-band millimetre camera, we mapped a field of
~ 90 arcminutes^2 in the direction of the star GJ526 simultaneously in its
1.15-mm and 2.0-mm continuum wavebands to investigate the nature of the
quasi-alignment of five sources found ten years earlier with the MAMBO camera
at 1.2 mm. We find that these sources are not clumps of a circumstellar debris
disc around this star as initially hypothesized. Rather, they must be
dust-obscured star-forming galaxies, or sub-millimetre galaxies (SMGs), in the
distant background. The new NIKA2 map at 1.15 mm reveals a total of seven SMGs
distributed in projection on the sky along a filament-like structure crossing
the whole observed field. Furthermore, we show that the NIKA2 and supplemental
Herschel photometric data are compatible with a model of the spectral energy
distributions (SEDs) of these sources when a common redshift of 2.5 and typical
values of the dust parameters for SMGs are adopted. Hence, we speculate that
these SMGs might be located in a filament of the distant `cosmic web'. The
length of this candidate cosmic filament crossing the whole map is at least 4
cMpc (comoving), and the separations between sources are between 0.25 cMpc and
1.25 cMpc at this redshift, in line with expectations from cosmological
simulations. Nonetheless, further observations to determine the precise
spectroscopic redshifts of these sources are required to definitively support
this hypothesis of SMGs embedded in a cosmic filament of dark matter.
