Monopolies are damaging to economies because they lead to higher prices. Monopolies are the only (or dominant) provider of a good or service, meaning that there is little to no competition in the market.
This gives the monopoly a competitive advantage because they hold most of the power.
Monopolies lead to higher prices because, without competition, a monopoly can set its own prices.
This is known as ‘price-fixing’ and it is a tool used by monopolies to maintain their profit margins by exploiting its power over the market and the consumer. In a monopoly, the absence of competition means that the consumers have no options other than the monopoly. Monopolies, therefore, retain the power to charge what they want because they know that the consumer has no other option but to pay the price charged by the monopoly. Monopolies are especially able to fix prices for products that people need, like gasoline.
As sole producers in a market full of consumers, monopolies hold control oversupply of the resource and use this to control prices. When demand is high they can limit supply output, making the resource more scarce and more valuable, thus driving up the price. This is done by profit-maximizing monopolies. A monopoly can decrease production in order to charge a higher price, which people will continue to pay because they are provided with no other alternatives. 
The monopoly’s control over the market constrains consumer choice and givers them the power to control the prices of goods and services. Doing so allows them to maximize profits by exploiting the market. With no alternative option, consumers must buy into the monopoly at the price they set. In a competitive market, competition ensures that producers offer lower prices to attract consumers. In a monopolized market, there is no competition and consumers have just one choice, one price point, and no alternatives.