Dark Matter Realism

Abstract

According to the standard model of cosmology, $$\Lambda $$ Λ CDM, the mass-energy budget of the current stage of the universe is not dominated by the luminous matter that we are familiar with, but instead by some form of dark matter (and dark energy). It is thus tempting to adopt scientific realism about dark matter. However, there are barely any constraints on the myriad of possible properties of this entity—it is not even certain that it is a form of matter. In light of this underdetermination I advocate caution: we should not (yet) be dark matter realists. The “not(-yet)-realism” that I have in mind is different from Hacking’s (Philos Sci 56 (4), 555–581, 1989) anti-realism, in that it is semantic rather than epistemological. It also differs from the semantic anti-realism of logical empiricism, in that it is naturalistic, such that it may only be temporary and does not automatically apply to all other unobservables (or even just to all other astronomical unobservables, as with Hacking’s anti-realism). The argument is illustrated with the analogy of the much longer history of the concept of a gene, as the current state of the concept of dark matter resembles in some relevant ways that of the early concept of genes.

A Dialogue among Recent Views of Entity Realism

“…Calculations show that galaxies would have appeared differently if they had not contained a large amount of invisible, dark matter. However, as Niels Martens (2022) argues, the properties of this entity have not yet been defined well. We do not even know whether it is really matter or not, or whether the dark matter part of the universe consists of one or several types of entities.…”

Robustness and Dark-Matter Observation

“…If it exists, dark matter could be anything, insofar as it satisfies a minimum set of constraints based on current cosmological observations. Martens (2022) aptly describes this minimum set of constraints as the 'thin common core concept of dark matter': assuming general relativity, dark matter is a massive field that contributes ≈ 27% to the current total cosmic mass-energy budget, it primarily interacts with baryonic matter via the gravitational force, and if it is a particle, its mass is expected to be between 10 −3 − 10 7 eV. Hence, assuming that one wishes to maintain general relativity at low accelerations, the 'thin common core concept of dark matter' embodies the minimum set of constraints that every candidate model of dark matter needs to satisfy in order to be compatible with current cosmological evidence related to various 'dark phenomena'.…”

“…Martens (2022) aptly describes this minimum set of constraints as the thin common core concept of dark matter : assuming general relativity, dark matter is a massive field that contributes ≈ 27% to the current total cosmic mass-energy budget; it primarily interacts with baryonic matter via the gravitational force; and if it is a particle, its mass is expected to be between 10 −3 and 10 7 eV. Hence, assuming that one wishes to maintain general relativity at low accelerations, the thin common core concept of dark matter embodies the minimum set of constraints that every candidate model of dark matter needs to satisfy in order to be compatible with current cosmological evidence related to various “dark phenomena.” 1 The thin common core concept of dark matter is therefore “common” because it is shared by all possible models for the nature of dark matter, and it is “thin” because it places very few constraints on the exact nature of dark matter.…”

The Virtues of Pursuit-Worthy Speculation: The Promises of Cosmic Inflation

“…3 an entrenchment desideratum (see Duerr and Wolf (2023) for details). This doesn't mean, of course, that the Dark Matter problem doesn't present us also with other forms of underdetermination-especially those concerning the physical nature of Dark Matter (Martens 2022).…”