My question follows on from the story in the last podcast about observing antimatter. How do we know that all the galaxies we see in the universe are made of matter and that some of them are not made of antimatter? Is the light generated by antimatter stars different to that generated from stars made of the same matter found in our galaxy? Gravitational effects are expected to be the same. So how do we know that there is an imbalance of antimatter and matter in the observable universe?