How can we tell the difference between matter and antimatter by observation in space? I just was wondering and searching on the internet with little luck in the topic. On Antimatter Wiki they tell the observable universe is built up by matter. I read antimatter can be detected in cosmic rays in very small quantity. But let's go in theories and our practical knowledge, my question is:

How could we detect if a set of objects (like an entire galaxy) is completely built up by antimatter? Since matter and antimatter look like "mirror image" on current, could they form a similar object which is indistinguishable from "our" matter objects? I mean, would spectrum analysis for example show that it is not matter, but built up by antimatter?

Linked question: Are we sure about that what we see in universe is matter?
Tl;DR


Detection via polarized light - Antimatter interaction with polarized light could be detected by vector rotation;
We're mostly sure, because absence of gamma rays and characteristic Faraday polarization indicates absence of observable antimatter in meaningful amounts.


Long answer

I do believe @userLTK to be correct on his comment.

To my limited knowledge, the absence of gamma-ray bursts that characterizes matter-antimatter interactions hints of baryonic matter prevalence throughout the visible universe; an anti-galaxy near a baryonic galaxy would show a healthy amount of gamma rays originating from boundary particle collisions.



This effect would be observable between structures of any scale - e.g. star/anti-star, galaxy/anti-galaxy or supercluster/anti-supercluster.

"Light from the Depths of Time", by Rudolph Kippenhahn, mentions that


  [...] Even when, like a star, it is radiation, its spectrum remains
  exactly the same, quite independent of whether atoms or anti-atoms are
  responsible for the light.


So the light originating from anti-galaxies would look the same as ordinary galaxies. But what about gravity?

A quote from "Isodual Theory of Antimatter: with applications to Antigravity, Grand Unification and Cosmology" may help us further:


  [...] However, the photons is invariant under charge conjugation and
  travel at the maximal causal speed in vacuum, c. Therefore, the
  photon could well result to be a superposition of positive and
  negative energies, perhaps as a condition to travel at the speed c,
  in which case the photon would be an isoselfdual state, thus
  experiencing attraction in both fields of matter and antimatter.


Meaning that antimatter would also cause gravitational effects, such as lensing.

That would make remote detection of anti-matter quite hard - it would emit and bend light in exactly the same fashion as common matter.

Faraday polarization rotation could give us some hope:


  Polarized light, e.g., from non-thermal synchrotron sources, that
  passes through gas with a non-zero magnetic field will have its
  polarization vector rotated by the process of Faraday rotation. [...]
  Note that regions dominated by antimatter (positrons) cause a rotation
  opposite to that caused by regions dominated by matter (electrons).


(SLAC Summer Institute on Particle Physics (SSI04), Aug. 2-13, 2004)

But the same source mentions that 


  The effect has been measured many times and the amount of rotation,
  usually expressed in terms of the so-called “rotation measure” is
  given by the line-of-sight integral [...] The fact that we observe an
  effect at all means that on average, we can’t have equal amounts of
  antimatter and matter along our various lines of sight.


So it seems that the visible universe is missing some antimatter after all.