I read that there are 300K immigrants from the former Soviet Union who are not recognized as being Jewish. This undoubtedly includes many who are but can’t prove it. It was very advantageous to be categorized as a Russian on one’s identity card (Jewish was considered a separate nationality along with Ukrainian, etc.). As they generally have very small families and have formed a separate sub-culture (many times I hear teenagers speaking Russian to each other, unlike the children of Jewish olim who speak to each other in Hebrew even if they speak to their parents in another language) the problem will eventually go away. Moslems, Xtians and Druze do not marry out.