The best way to fill in that sentence if you need two words is to use the phrase “natural monopoly.” A single seller in a market where fixed costs are so high that only one seller can survive is said to enjoy a natural monopoly.
The classic examples of natural monopolies are found in public infrastructure like electricity or water. To deliver water to every home in a city takes a huge amount of infrastructure. Pumping stations must be built. Pipes must be buried in huge networks. These are fixed costs that exist regardless of how many customers the company serves. It would be very much more expensive for customers if competing water companies each built their own networks. For this reason, water is generally provided by a monopoly, often owned by the government. This sort of firm is known as a natural monopoly.