Many physicists will happily tell you that the universe is primarily made up of a substance called 暗物质, but ask for a definite answer on why we haven't directly glimpsed it yet and you would be met with a blank face. Researchers in the US, however, believe that the answer could lie in remnants of the collision of two galaxy clusters, where they may have caught a glimpse of 暗物质 interacting via a long-range "第五力量". If the 第五力量 does exist, it would require a major revision of the current Standard Model of particle physics, which has been with us for nearly 30 years (物理莱特牧师 98 171302）。
美国纽约大学的Glennys Farrar和Rachel Rosen认为，这种证据的证据“fifth force”可以通过研究约30亿光年远的两个单独星系团之间的碰撞来找到。这次冲突被昵称为“子弹群”，由一个较小的“bullet”星系团，它已经通过了一个特别大的星系团。
The trick to their reasoning is to place plausible limitations on what the initial approach speed and mass distribution of the 子弹 could have been, given what astronomers already know about the dynamics of other galaxy clusters. Then, by recording how fast the 子弹 is moving away from the larger cluster, they would be able to deduce whether the acceleration is the value that would be produced by gravity alone acting on 暗物质.
Farrar和Rosen已经使用钱德拉卫星拍摄的X射线数据测试了他们的理论，这表明“shock front” of the 子弹 to be 4740 km/s. This speed is too high to have been produced by gravity alone, implying that gravity acting on the 暗物质 in the 子弹群 is being augmented by a 第五力量 between 0.4 and 1.2 times as strong. Such a 第五力量, if it does exist, would be proof that the Standard Model is incomplete —可能需要诸如超对称之类的扩展，其中包括附加的承载力粒子。
However, the physicists now think that the speed of the 子弹 given by the Chandra data may not be quite right. In particular, Farrar believes that speed recorded was of the 冲击前 relative to the surrounding gas, which is falling into the cluster. This means the data produced an estimate of the overall speed of the collision too high by some 1500 km/s (arXiv.org/astro-ph/0703232).
法拉尔告诉 物理网 that it is too soon to know whether this will rule out the existence of a 第五力量, as she will now have to reanalyse the 子弹群 using the updated measurements. Nevertheless, Douglas Clowe, a cosmologist from Ohio University in the US who has spent much time searching for evidence of 暗物质 in the 子弹群, says that the test for the 第五力量 laid out by Farrar and Rosen will still have importance in the future. “Whether a 第五力量 is [ultimately] measured or we put a limit on how strong it could be will help in our continuing search for what 暗物质 actually is –但它不太可能是识别它的最终作品。”