There is such a thing as radioactive decay. It's a very widely studied and replicated phenomenon, and is used in many practical applications (e.g. smoke detectors). It's about as close to objective fact as any scientific result can get, and that will not change.
Perhaps you mean that radioactive decay may have a causal mechanism involving dark matter? Possibly. For such a model to be taken seriously, it would need to agree with our (presumably very numerous and precise) existing observations, as well as either predict the outcome of some new, falsifiable experiment; and/or simplify/unify some disparate existing results into a common framework.
Even if such a model manages to overcome all of those filters, radioactive decay would still be a very real phenomenon. For example, discovering that light is an electromagnetic wave doesn't at all imply that "there's no such thing as light"; discovering that gravity is curvature of spacetime doesn't mean "there's no such thing as gravity"; etc. Like radioactive decay, those are phenomena which we have very direct evidence for, and which theories and models must try to explain.
In comparison, some theories/models postulate the existence of entities which we have no direct evidence for, such as phlogiston, luminiferous aether, dark matter, etc. The only evidence for these is the success of the model that predicts them; that evidence may be undermined if a better model is discovered which doesn't require these entities (e.g. the kinetic theory of gases for phlogiston and special relativity for luminiferous aether).
Hence, if we discover a better model for predicting flat galactic rotation curves, gravitational lensing in excess of the visible matter, etc. which doesn't require an exotic form of matter, then we might claim "there's no such thing as dark matter".
However, even if that were to happen, we certainly couldn't claim that "there's no such thing as flat galactic rotation curves", "there's no such thing as gravitational lensing in excess of the visible matter", etc. since, like radioactive decay, those are phenomena we have observed.
If a theory disagrees with observed phenomena, or claims "there's no such thing" as the observed phenomena, then the theory is wrong. That's precisely what empirical falsification is about. We've observed the phenomena, so that theory cannot describe the world we live in.
You're taking it way too literally, I think it was clear what I meant. I meant if the decays of atoms could be the result of interactions with the particles of dark matter, rather than spontaneous decay caused by random quantum fluctuations. If it's true, we should be able to observe differing rates of decay depending on the concentration of dark matter in the area.
You wrote "What if there's no such a thing as <something we have studied and understood in detail for decades> and we've been observing interactions with <something we hypothesise might exist but we know next to nothing about> all along?"
Why did you jump to the conclusion that the well understood thing should be replaced by the new and unknown thing?
It doesn't seem to me like a big leap of logic to go from something being shown to be influenced by a so far unknown particle and the same thing being the result of interactions with this particle.
Perhaps you mean that radioactive decay may have a causal mechanism involving dark matter? Possibly. For such a model to be taken seriously, it would need to agree with our (presumably very numerous and precise) existing observations, as well as either predict the outcome of some new, falsifiable experiment; and/or simplify/unify some disparate existing results into a common framework.
Even if such a model manages to overcome all of those filters, radioactive decay would still be a very real phenomenon. For example, discovering that light is an electromagnetic wave doesn't at all imply that "there's no such thing as light"; discovering that gravity is curvature of spacetime doesn't mean "there's no such thing as gravity"; etc. Like radioactive decay, those are phenomena which we have very direct evidence for, and which theories and models must try to explain.
In comparison, some theories/models postulate the existence of entities which we have no direct evidence for, such as phlogiston, luminiferous aether, dark matter, etc. The only evidence for these is the success of the model that predicts them; that evidence may be undermined if a better model is discovered which doesn't require these entities (e.g. the kinetic theory of gases for phlogiston and special relativity for luminiferous aether).
Hence, if we discover a better model for predicting flat galactic rotation curves, gravitational lensing in excess of the visible matter, etc. which doesn't require an exotic form of matter, then we might claim "there's no such thing as dark matter".
However, even if that were to happen, we certainly couldn't claim that "there's no such thing as flat galactic rotation curves", "there's no such thing as gravitational lensing in excess of the visible matter", etc. since, like radioactive decay, those are phenomena we have observed.
If a theory disagrees with observed phenomena, or claims "there's no such thing" as the observed phenomena, then the theory is wrong. That's precisely what empirical falsification is about. We've observed the phenomena, so that theory cannot describe the world we live in.