How do we know how much visible matter exists?

[Xezlec]

In discussions of how we infer the existence of dark matter, it always feels like people gloss over how exactly we know how much "visible matter" exists. I am led to believe that "visible matter" refers to any matter which can interact with things like photons, but doesn't that include things that might not be bright and flamey like stars? How can we tell there isn't just way more dirt, dust, gas, rock, rogue planets, space squirrels, icky black crud, and floating black velvet paintings of Elvis out there than what is close to stars and thereby illuminated?

 

[malawi_glenn]

You are referring to Baryonic Dark Matter:

http://en.wikipedia.org/wiki/Baryonic_dark_matter

http://www.astro.princeton.edu/~dns/MAP/Bahcall/final.html

 

[Math Is Hard]

You are referring to Baryonic Dark Matter:

http://en.wikipedia.org/wiki/Baryonic_dark_matter

http://www.astro.princeton.edu/~dns/MAP/Bahcall/final.html

No, I think the question is about plain old baryonic matter. Not dark. It's a good question.

 

[malawi_glenn]

No, I think the question is about plain old baryonic matter. Not dark. It's a good question.

It depends on how you define "dark matter". The proposed explanations from Xezlec are within the definitions on baryonic dark matter, (MACHO). The deifinition made by wiki article on Dark Matter "dark matter is hypothetical matter that does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter." which already in the definition out rules the possibility of baryonic (in this context non leptonic, neutrino background are referred to as baryonic dark matter even thoug neutrinos are leptons)- and strong interacting dark matter.

The best definition of dark matter would be matter that can't be observed by telescopes due to their EM interactions but must exist due to its gravitational interaction. With this definition; both WIMPs, MACHOs, 'baryonic' and SIMPs (strongly interacting dark matter) are possible.

 

[Xezlec]

Thanks, everyone! That answers my question six ways from Sunday, and gives me more to read about too.