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James Weatherall, Visiting Professor of Philosophy at Stanford (Winter 22) University of California, Irvine

Wed March 9th 2022, 4:00 - 6:00pm

The Philosophy Behind Dark Matter

According to the Standard Model of Cosmology, more than 80% of the total matter in the universe is “dark”: it does not emit or reflect light, and so its presence and properties can be inferred, at least thus far, only through its gravitational influences. The inference from astrophysical and cosmological observations to the existence of dark matter has attracted considerable scrutiny from philosophers and philosophically-minded physicists, many of whom have argued that the case for dark matter is weaker than astrophysicists often suggest. Such authors tend to emphasize that there are plausible theoretical alternatives to dark matter available, suggesting that dark matter should be viewed as a tentative hypothesis whose status within the astrophysics and cosmology communities is not supported by the available evidence. In this talk, I will assess the current state of evidence for dark matter and argue that today, the case for dark matter is extremely strong — comparable to, or stronger than, the case for many other “unobservable” entities in 20th and 21st century physics. But as I will discuss, astrophysicists appear to have accepted dark matter as the most plausible hypothesis long before the most convincing contemporary evidence became available — suggesting that in an earlier phase of research, during the 1980s and 1990s, the sort of underdetermination noted by philosophers may have obtained after all. I will argue that physicists’ preference for dark matter during this period was influenced by their widespread commitment to certain views that might be thought of as a “philosophy of science”, concerning explanation, the nature of scientific theories, and principles of action and reaction (or cause and effect). I will then suggest that, in light of subsequent evidence, these philosophical commitments led physicists in the right direction, and address whether they can be expected to do so in other contemporary cases where the evidential situation is less clear.