Recent precise observations of the 2.7 K CMB by the Planck mission toward the Coma cluster are not in agreement with X-ray measurements. To reconcile both types of measuring techniques we suggest that unstable dark matter is the cause of this mismatch. Decaying dark matter, which gravitationally dominates the galaxy cluster, can affect the estimated hot plasma content, which is then missing in the measured SZ effect from exactly the same place in the sky. The model independent lifetime of dark matter decaying entirely to X-rays is estimated to be about 6x10^{24} sec; this lifetime scales down with the fraction of the radiatively decaying dark matter. In addition, it is shown that the potential of such dark matter investigations in space is superior to the largest volume Earth-bound dark matter decay searches. Other clusters might provide additional evidence for or against this suggestion.
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