  (abridged) Infrared excesses associated with debris disk host stars detected
so far peak at wavelengths around ~100{\mu}m or shorter. However, six out of 31
excess sources in the Herschel OTKP DUNES have been seen to show significant -
and in some cases extended - excess emission at 160{\mu}m, which is larger than
the 100{\mu}m excess. This excess emission has been suggested to stem from
debris disks colder than those known previously. Using several methods, we
re-consider whether some or even all of the candidates may be associated with
unrelated galactic or extragalactic emission and conclude that it is highly
unlikely that none of the candidates represents a true circumstellar disk. For
true disks, both the dust temperatures inferred from the SEDs and the disk
radii estimated from the images suggest that the dust is nearly as cold as a
blackbody. This requires the grains to be larger than ~100{\mu}m, regardless of
their material composition. To explain the dearth of small grains, we explore
several conceivable scenarios: transport-dominated disks, disks of low
dynamical excitation, and disks of unstirred primordial macroscopic grains. Our
qualitative analysis and collisional simulations rule out the first two of
these scenarios, but show the feasibility of the third one. We show that such
disks can survive for gigayears, largely preserving the primordial size
distribution. They should be composed of macroscopic solids larger than
millimeters, but smaller than kilometers in size. Thus planetesimal formation,
at least in the outer regions of the systems, has stopped before "cometary" or
"asteroidal" sizes were reached.
