For the last couple of years librarians have talked about a 20th century black hole when trying to describe the effect that copyright has on making cultural heritage available online (it appears that the concept was first used publicly by Prof. James Boyle in a 2009 column for the Financial Times).
At Europeana we are able to show the 20th century black hole in our dataset by looking at the temporal distribution of works within the dataset. We did so in a first analysis that we undertook in May 2012 and we have just repeated this exercise at the request of the European Commission which is looking for evidence to assess the impact of copyright on the online availability of cultural heritage. Just as in 2012 we are seeing the concept of the 20th century black hole confirmed in our data.

Source : The missing decades: the 20th century black hole in Europeana – Europeana Professional