Dark fiber is fiber optic cable that has been laid, but not in use. It is called dark fiber because fiber optic cables transmit pulses of light instead of electrical current, so hence the "dark". There is so much of it because in the internet boom of the late 1990's, companies were spending loads of money building out fiber networks and when the bubble burst, some of these companies went bankrupt. Other's just didn't have the demand needed to utilize all of this fiber capacity.
Dark fiber isn't the problem, what is the issue is connecting the individual homes and businesses to fiber networks, the so-called "last mile" problem. It does cost a lot of money to bring fiber to the premesis, but Verizon and other companies have promised fiber to the home for years and are just starting to connect people up.
Google is a player with dark fiber since it can be bought for pennies on the dollar for the cost of building these networks. Who knows what Google is up to? Some people think they will roll out their own internet service and others think they just are connecting various data centers they control.

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