"According to general relativity, mass "warps" space-time to create gravitational fields and therefore bend light as a result. This theory was confirmed in 1919 during a solar eclipse, when Arthur Eddington observed the light from stars passing close to the sun was slightly bent, so that stars appeared slightly out of position." - Wikipedia article "Gravitational lens"

 i hear all the time that it's the dark matter within galaxies that causes gravitational lensing of other galaxies. if the sun can cause gravitational lensing, why can't it be true that it's just the combined mass of all the matter in a galaxy that causes the lensing, and not necessarily dark matter?

Because the amount of warpage which is observed is far greater than what can be accounted for by the visible mass in those galaxies.  There must be something we cannot see adding the the gravitational effect.

 aha! thanks for the answer, zachsdad.

 so i was recently reading a thread in the cosmology section of the forums about the dark matter/H2 debate. ive never really agreed with the theory of dark matter, and i think the molecular hydrogen side of the debate is a more believable alternative. any thoughts?

That molecular H2 argument sounds pretty plausible until you realize that the amount of hydrogen we can account for matches very well what the BBT predicts.  So, where did all this extra H2 come from?  We are talking about more H2, in mass, than all the visible material in the universe.  The BBT would have to be waaaay off.  Those same calculations nail the amount of helium very well.

 any chance there's some nearly invisible helium out there too?

RIP_Shadowfox:

 any chance there's some nearly invisible helium out there too?

In my humble opinion, no.  I don't have any problem with the concept of Dark Matter.  I just see it as something which has an effect we can measure, but we just can't understand it yet.  Heck, we are in the same boat with gravity itself, we know it exists (with all the snow and ice this winter I'm very aware of gravity's potential to demonstrate itself to me every time I walk outside ) but we don't really know what it is, yet.  Dark Matter is not magic, it's just shy.

What are the possibilities that dark matter is a consequence of higher dimensional events, that of which we can't perceive?

I'm a firm believer in multiverse theory, and I am fascinated with the theory that some physicists have proposed that gravity actually propagates through as many as 11 dimensions, and since we only perceive 3 spatial dimensions, we perceive gravity as being somewhat weak.

Could the same go for dark matter? Is it possible we are experiencing a phenomenon that's not entirely of our 3-dimensional spatial universe?

It's my opinion that Dark Matter and Dark Energy are firmly rooted in this universe.  We are still in our technological infancy, as we develop better tools we'll better understand the workings of the cosmos. 

I don't spend much time thinking about the multi-verse possiblity because even if there are a million other universes 'out-there' we will never be able to know them, or even to sense them, so their existance is entirely speculation and mathematical gymnastics.  There's plenty in this universe to break my brain over.

zachsdad:

or even to sense them,