Islam can be found in the old world and the new.
Muslims are present in every race, ethnicity and country.
Over the centuries the borders of the Muslim world have shifted somewhat, particularly with regards to Europe. During certain periods of history many parts of Europe were within the borders of the Muslim world; including Spain, some of southern France, all Mediterranean Islands, parts of Italy, parts of Central Europe, most of Southeast Europe, Russia, and Ukraine.
Muslims came to the New World at pretty much the same time as Columbus (some of his captains and other crew may have been Muslims) and possibly well before. Indeed Muslims of many ethnicities have been in the Americas for centuries.
But the heartland of Islam has always been the heartland of humanity. That is, the central or middle parts of the world—with Arabia being the home of the Prophet Muhammad. Today the predominantly Muslim countries range from Morocco in the West and from there moving eastward through North and West Africa to Arabia, then continuing through Central Asia to the Indian Sub-continent and finally to the East Indies.
Most Muslims are actually not Arabs. Indeed countries with highest Muslim populations are non-Arab: Indonesia (with 12.7% of all Muslims in world), followed by Pakistan (11%) and then India (10.9%)—the last is not even a Muslim country.
But significant Muslim populations can be found all over the globe. Millions of Muslims inhabit Europe and North America, where foreigners worship side-by-side with growing native populations. There are eleven million Muslims in China where one of the oldest mosques in the world—older than most mosques in the Muslim heartland—is still in use.
Islam is often cited as the fastest or one of the fastest growing religions in the world. It is projected to be the largest religion in the world 50 years from now. Though most of this growth is due to high birth rates, Islam also has possibly the highest conversion rate of all religions. Most predominantly non-Muslim nations the world over are seeing conversions to Islam among their populations.