Andromeda is made of antimatter. Am I wrong? Why? Andromeda is made of antimatter. Am I wrong? Why?  
Of course I do not know that Andromeda is made of antimatter.
_but____ I do not know that Andromeda is made of matter. 
Does anybody know what is the correct sentence?  Why?
EDIT
I used Andromeda as a substitute for any non-local galaxy.
Lets put some geometry (due to the first two comments):
Antimatter galaxies (AMGs) have in general AMGs in the neighborough. MGs only MGs.
For instance suppose the hurricanes in the northern hemisphere rotate X way and in the southern rotate anti-X way, and they dont meet ;).
That is to say, it is not mandatory that AMGs colide at all with MGs.  
I suspect that we have no way to decide if the light we receive is originated in an anti-atom or from an atom.  
 A: I believe the current modern theory is that there cannot exist anti-matter galaxies unless anti-matter is discovered to have a repulsive interaction with matter via gravity (as opposed to the normal attractive force), which would have interesting applications to the structure of the universe. However, to my understanding, this is not generally theoretically supported as most models have anti-matter interacting normally, and also if Andromeda was an anti-matter galaxy then it would not be colliding with the Milky Way anytime soon.
If there were anti-matter galaxies with anti-matter behaving like matter gravitationally then we would occasionally see HUGE explosions across the sky as anti-matter galaxies collide with matter galaxies and anti-matter stars collide with matter stars. 
Just some quick napkin calculations: lets take two solar mass stars, one matter and one anti-matter and see how much energy is released when they annihilate. $2M = 4 \times 10^{30}$ so since $E=mc^2$ we have $E= 3.6 \times 10^{47}$ joules which is about four times the energy released by the most intense gamma ray burst. 
Although the dynamics would be interesting as I wonder how much a repulsion effect annihilations with inter-stellar medium would be, as well as wether or not when the two stars start to annihilate they would "explode" away from each other...
A: IF Andromeda were antimatter, or any galaxy in general, we would be able to notice the huge amount of energy from colliding matter and antimatter galaxies. As we don't, the only explanation is (while still thinkin' "Andromeda is antimatter") that matter and antimatter would repel each other instead of having normal gravity. However, Andromeda IS going to collide with milky way and if the un-gravity theory were true, it would not be happening.
A: One way of looking at this is thru time. Andromeda is either "made of matter" or "made of antimatter" if we experience it using a direction of time we exhibit on our galaxy. If we experience it in reverse time, it changes from matter to antimatter and vice versa.
So the question is whether the direction of time is absolute. If it's not, Andromeda is either made of matter or antimatter, depending on the direction of time of the observer.
