10th Dennis Sciama Memorial Lecture by Prof. James Binney. Cosmology tells us that most “ordinary” matter such as we are made of is not in stars or in the interstellar media of galaxies. So it must lie between galaxies. In rich clusters of galaxies it is so dense and so hot that its thermal X-ray emission has long been detected. But cluster galaxies have long had very low star-formation rates, while field galaxies like ours have continued to form stars even though the surrounding intergalactic medium is too rarefied to be detected. Chemical signatures indicate that our Galaxy has continued to accrete relatively pristine gas but there is much evidence that star formation leads to efficient ejection of gas from galaxies. A picture will be assembled of how galaxies like ours exchange matter with the intergalactic medium. This exchange influences the radial distribution of star formation and implies a specific role of massive black holes in galaxy evolution.