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> I look at dark matter/energy as a measurement of what we don't know - it is not necessarily matter or energy (although it could be). Dark matter is called ma

> I look at dark matter/energy as a measurement of what we don't know - it is not necessarily matter or energy (although it could be). Dark matter is called matter because it has mass, i.e. it experiences gravitational attraction to itself and to other forms of matter. Dark energy is called energy because it acts like radiation, i.e. it has negative pressure. > Personally, I don't like dark matter because my "hack" alarm bells start ringing. That being said, I also don't know what I don't know. Your second sentence is far closer to the mark than your first. Dark matter was indeed seen as a hack in the 1930s when it was first proposed, but there is now a large and remarkably consistent body of observations that show its presence. Here's one lay-friendly blog post that briefly describes some of the evidence: https://medium.com/starts-with-a-bang/why-dark-matter-9252bfa21a8a https://medium.com/starts-with-a-bang/why-dark-matter-9252bf... > In that way, it wouldn't hurt if slightly fewer eyes were looking at particle dark matter and slightly more at "non-matter" dark matter. It's kind of like quantum mechanics at this point. The only reason there aren't more eyes looking at alternatives to dark matter is because we already did that over the last few decades, and the evidence in favor of dark matter kind of became ridiculously overwhelming. If you want to float an alternative to dark matter at this point, you have to, at the very least, come up with a theory that explains all the things that dark matter explains. (Presumably you would also come up with something that dark matter doesn't explain that your theory does, but one thing at a time.)

chriswarbo 10y ago
> Dark matter is called matter because it has mass, i.e. it experiences gravitational attraction to itself and to other forms of matter. > Dark energy is called energy because it acts like radiation, i.e. it has negative pressure. I was going to post something similar, but tempered a little, since some participants in this discussion are skeptical of the existence of these things. I think the dark energy statement is fine, but would weaken the dark matter one to say, e.g., it's called matter because we can observe effects (lensing, rotation curves, etc.) which can be explained by the presence of matter. In this way, dark matter and dark energy definitely exist, in the same way that centrifugal and coriolis forces definitely exist: they explain concrete observations, and by squinting at the formulae we see that the terms we've introduced have units of mass (dark matter), energy (dark energy) and force (centrifugal and centripetal). The effects explained by centrifugal and coriolis forces can also be explained in a different way, by taking into account the rotation of our frame of reference. Articles like this one show that the jury's still out on whether the effects of dark matter and dark energy can be explained in a different way. AFAIK dark matter is quite widely accepted as a fundamental explanation, rather than a "fictitious" one like centrifugal/coriolis forces, while it's common to treat dark energy as a "placeholder" at the moment: it may be a form of energy which permeates space, or it may be an effect of something else (e.g. some unexpected behaviour of spacetime, like discretisation or some other change of metric). While we're still some way from understanding dark energy, I have no problem with models of dark matter (although SUSY's looking less likely every day); after all, why should we demand that all matter interact elecromagnetically? On the other hand, when theorising about what dark energy might be, I think it's fine if that explanation encompasses the effects of dark matter too. Just because we discovered the dark matter effects first, it doesn't mean they must be explained first.

chriswarbo 10y ago

> Dark matter is called matter because it has mass, i.e. it experiences gravitational attraction to itself and to other forms of matter. > Dark energy is called energy because it acts like radiation, i.e. it has negative pressure. I was going to post something similar, but tempered a little, since some participants in this discussion are skeptical of the existence of these things. I think the dark energy statement is fine, but would weaken the dark matter one to say, e.g., it's called matter because we can observe effects (lensing, rotation curves, etc.) which can be explained by the presence of matter. In this way, dark matter and dark energy definitely exist, in the same way that centrifugal and coriolis forces definitely exist: they explain concrete observations, and by squinting at the formulae we see that the terms we've introduced have units of mass (dark matter), energy (dark energy) and force (centrifugal and centripetal). The effects explained by centrifugal and coriolis forces can also be explained in a different way, by taking into account the rotation of our frame of reference. Articles like this one show that the jury's still out on whether the effects of dark matter and dark energy can be explained in a different way. AFAIK dark matter is quite widely accepted as a fundamental explanation, rather than a "fictitious" one like centrifugal/coriolis forces, while it's common to treat dark energy as a "placeholder" at the moment: it may be a form of energy which permeates space, or it may be an effect of something else (e.g. some unexpected behaviour of spacetime, like discretisation or some other change of metric). While we're still some way from understanding dark energy, I have no problem with models of dark matter (although SUSY's looking less likely every day); after all, why should we demand that all matter interact elecromagnetically? On the other hand, when theorising about what dark energy might be, I think it's fine if that explanation encompasses the effects of dark matter too. Just because we discovered the dark matter effects first, it doesn't mean they must be explained first.