  We constrain the physical nature of dark matter using the newly identified
massive merging galaxy cluster MACSJ0025.4-1222. As was previously shown by the
example of the Bullet Cluster (1E0657-56), such systems are ideal laboratories
for detecting isolated dark matter, and distinguishing between cold dark matter
(CDM) and other scenarios (e.g. self-interacting dark matter, alternative
gravity theories). MACSJ0025.4-1222 consists of two merging subclusters of
similar richness at z=0.586. We measure the distribution of X-ray emitting gas
from Chandra X-ray data and find it to be clearly displaced from the
distribution of galaxies. A strong (information from highly distorted arcs) and
weak (using weakly distorted background galaxies) gravitational lensing
analysis based on Hubble Space Telescope observations and Keck arc spectroscopy
confirms that the subclusters have near-equal mass. The total mass distribution
in each of the subclusters is clearly offset (at >4sigma significance) from the
peak of the hot X-ray emitting gas (the main baryonic component), but aligned
with the distribution of galaxies. We measure the fractions of mass in hot gas
(0.09^{+0.07}_{-0.03}) and stars (0.010^{+0.007}_{-0.004}), consistent with
those of typical clusters, finding that dark matter is the dominant contributor
to the gravitational field. Under the assumption that the subclusters
experienced a head-on collision in the plane of the sky, we obtain an
order-of-magnitude estimate of the dark matter self-interaction cross-section
of sigma/m < 4cm^2/g, re-affirming the results from the Bullet Cluster on the
collisionless nature of dark matter.
