They are dangerous for women who are pregnant, because of the unknown defects caused by radiation leakage. Metal containers should be avoided because they reflect microwaves, and prevent the microwaves from entering the food. Securing door seals and proper door closure helps to prevent microwave leakage. Microwave ovens that don't close securely should never be used. Microwaves are not known to be produced by any celestial bodies.
Microwaves are a type of electromagnetic energy like light waves.
The one difference is that microwaves cannot be seen by the human eye. Microwaves have longer wavelengths than light waves. Like light waves, microwaves may be reflected and concentrated. Microwaves pass easily through rain, smoke, and fog which block lightwaves. High altitude power microwaves can be developed and used for communication and laser beam direction.