This is not, as I often do, a map that has been done by someone else, but rather raw data from Eurostat that I have represented. Some months ago I commented on a project concerning a population grid, 1 km wide, covering the whole of Europe, as to give a better vision on some issues, as population, whose rendering following administrative basis was far from good.
So, there I went to the Eurostat specific site ((http://epp.eurostat.ec.europa.eu/portal/page/portal/gisco_Geographical_information_maps/popups/references/population_distribution_demography) to download the GEOSTAT 1 km2 population grid, with associated 2006 population data. The density map is somehow known as we know the main cities and axis, but what is less known is the map of the void spots (in fact, Eurostat does not produce a polygon for those 1 sq m cells without residents). As often for European data, there are countries out of the Union (Switzerland, Norway, Iceland) that are represented, while others (Cyprus) are not there).
The available cells (the populated ones, almost 2 million) help get the voids by exclusion; at first glance you can see substantial void areas in Spain, the Alps, the Charpatians, parts of Greece and the Scottish and Scandinavian mountains.
But it is far more interesting to better portray the empty areas.