Dark Matter Candidates 213Another major cluster merger is DLSCL J0916.2+2951 [17] at푧=0.53, in which the
collisional cluster gas has become clearly dissociated from the collisionless galaxies
and dark matter. The cluster was identified using optical and weak-lensing observa-
tions as part of the Deep Lens Survey. Follow-up observations with Keck, Subaru,
Hubble Space Telescopes, and Chandra show that the cluster is a dissociative merger
which constrains the DM self-interaction cross-section to휎푚−DM^1 ⩽7cm^2 /g. The system
is observed at least 0. 7 ± 0 .2 Gyr since first pass-through, thus providing a picture of
cluster mergers 2–5 times further progressed than similar systems observed to date.
9.5 Dark Matter Candidates
If only a small percentage of the total mass of the Universe is accounted for by stars
and hydrogen clouds, could baryonic matter in other forms make up DM? The answer
given by nucleosynthesis is a qualified no: all baryonic DM is already included in훺b.
Dark Baryonic Matter. Before the value of훺dmwas pinned down as certainly as it
is now, several forms of dark baryonic matter was considered. Gas or dust clouds were
the first thing that came to mind. We have already accounted for hot gas because it is
radiating and therefore visible. Clouds of cold gas would be dark but they would not
stay cold forever. Unless there exists vastly more cold gas than hot gas, which seems
unreasonable, this DM candidate is insufficient.
It is known that starlight is sometimes obscured bydust, which in itself is invis-
ible if it is cold and does not radiate. However, dust grains re-radiate starlight in
the infrared, so they do leave a trace of their existence. But the amount of dust and
rocks needed as DM would be so vast that it would have affected the composition
of the stars. For instance, it would have prevented the formation of low-metallicity
(population-II) stars. Thus dust is not an acceptable candidate. And baryons are just
too few to explain DM.
Snowballsof frozen hydrogenic matter, typical of comets, have also been consid-
ered, but they would sublimate with time and become gas clouds. A similar fate
excludescollapsed stars: they eject gas which would be detectable if their number
density were sufficient for DM.
A more serious candidate for baryonic matter has beenjupitersorbrown dwarfs:
starsofmasslessthan0. 08 푀⊙. They also go under the acronym MACHO forMas-
sive Compact Halo Object. They lack sufficient pressure to start hydrogen burning,
so their only source of luminous energy is the gravitational energy lost during slow
contraction. Such stars would clearly be very difficult to see since they do not radi-
ate. However, if a MACHO passes exactly in front of a distant star, the MACHO would
act as a gravitational microlens, because light from the star bends around the massive
object. The intensity of starlight would then be momentarily amplified (on a timescale
of weeks or a few months) by microlensing, as described in Section 4.3. The problem
is that even if MACHOs were relatively common, one has to monitor millions of stars
for one positive piece of evidence. Only a few microlensing MACHOs have been dis-
covered in the space between Earth and the Large Magellanic Cloud, but not enough
to explain DM.