Blue-ward of the Lyman limit, the sky is pretty much opaque, beyond a few pc (except in one or two directions, such as the http://chandra.harvard.edu/photo/2005/bhlock/"), until well into the soft X-ray region, due to absorption by neutral H. H is a ubiquitous component of the ISM (and IGM). So is He. Are there two regions of the EM spectrum with similar 'fogs'? One corresponding to absorption by neutral He, the other by He+? I did a quick google search, but didn't turn up anything in the way of papers reporting good observations.