Faith in the existence of dark matter.
In 1933 Fritz Zwicky was studying a galactic cluster and observed that the rotational speed of galaxies in the cluster was impossible unless there existed a huge amount of unobserved mass in the cluster. He, following in the footsteps of Lord Kelvin 50 years earlier, called this unobserved mass "dark matter".
The Zwicky observations have been confirmed over and over again in other galactic clusters and in the late 70's Vera Rubin and Kent Ford published new observational results using spectrographic analysis to confirm that most galaxies must contain approximately 6 times as much "dark matter" as visible mass.
Over the last 40 years numerous experiments have been conducted to test the two main hypothesis about what sort of particle dark matter might be composed from, WIMPS and axions. All those experiments have failed to detect the existence of either particle.
Dark matter has never been directly observed. Astrophysicists do not know what dark matter is. They believe however, absent any direct evidence, that dark matter exists. They have it seems "a faith based belief in the existence of dark matter".
Physicists generally have enormous faith in the existence of this unobserved entity (some don't, there are alternative theories.) But their faith in the existence of dark matter is entirely different than faith in the existence of gods.
While there have been no direct observations of dark matter, as noted earlier "dark matter" itself is just a placeholder for a problem with repeatedly confirmed observational data. There is plenty of indirect evidence. Something is causing the observed problem, one set of theoretical explanations assume the existence of large quantities of mass from an unknown new type of particle.
Research institutes across the planet invest resources into the search for dark matter based entirely on faith that this research, even if it, as it has so far done, fails to find any evidence for the constituent particle or particles. Their research is probing into one of the great mysteries in human knowledge, it is research into the boundaries of the unknown. It is part of the great project of the enlightenment to understand the universe we inhabit.
While faith in the existence of dark matter is pervasive among astrophysicists, this belief is not "unshakable". In fact, quite the opposite. If one of the alternative theories, for example that galactic clusters contain enormous numbers of very small black holes, proves valid, the search for dark matter particles would likely come to an end. Unlike irrational faith, scientific faith is based on theory, observation experimentation and analysis. New evidence can obsolete current knowledge.
= new reply since forum marked as read
"a best guess placeholder." Perhaps. A bet on Pascal's Wager. 
...I wasn't really criticizing, I was likely doing something more akin to pontificating. 
In the hopes that I was clarifying and cutting-to-the-point.
I may have missed my mark.
