Neutral hydrogen 21 cm emission is observed in many late
 type spiral galaxies to extend far outside the optical disks.
 In addition, the rotation curves derived from the 21 cm line
 show in many such cases a constant rotational velocity persisting
 to large galactocentric radii, indicating the presence of large
 amounts of dark matter. Besides the massive halo picture,
 another possible form of dark matter is the very cold gas and dust
 in the disk of the galaxy. We propose to search for the presence
 of very cold dust ( <15 K) in the outer parts of the disk of
 the NGC 5097 spiral galaxy.

 ISOPHOT  has the capability to observe beyond the IRAS long-wave-
 length limit up to 240 microns and has an unprecedented sensitivi-
 ty in this waveband. Furthermore, the combination of PHT photometry
 in three wavebands, 180, 135, and 90 microns, enables us to
 eliminate the disturbing effects of warmer foreground dust
 (galactic cirrus) and to estimate the temperature of the very
  cold dust.