PubMed Central is a free database of scientific literature in biomedical and life sciences. It is on the Internet. It grew from the online Entrez PubMed biomedical literature search system. PubMed Central was developed by the U.S. National Library of Medicine (NLM) as an online archive of biomedical journal articles. It was started in 2000. As of 2021, it had over 7 million science papers in it.