The tip of the iceberg.
The tip of the iceberg, is one of the recurring theme that comes up during PI's public events. Almost each time, I hear how little we know about the physical world. Last year, for example, during the Einsteinfest intriguing Inventors, Inventions, Conquerors and Conquests - The Ingredients for Success Bistro Banter, when the debate about how much we know (minute 29:26) was on, an analogy was drawn between how little we know about universe (minute 32:58) and our knowledge of the bottom of our oceans (minute 30:21). Also, Lisa Randall during hers "Warped Passages" public lecture on March the 1st this year, said that "secrets of the Universe are about to unravel" and that there are "hidden riches in the Universe" (see below picture).
If we don't see the dark matter how do we know about it? In physics it is not unusual to know that something exists without seeing it. Take for example the magnetic/radioactive forces or electromagnetic waves. They are there but we don't see them. We experience though their presence. Sometimes such an experience could be quite helpful (as in the antimatter case).
Among the many explanations regarding dark matter's existence Damian used the Rubin-Ford experimental observations to convince us. I will explain next time how he did it.