Electromagnetic waves are vibrations of magnetic and electric fields.
It can be created by accelerating charges; moving charges back and forth will produce oscillating electric and magnetic fields, and these travel at the speed of light, because light is just one example of an electromagnetic wave.
Speed of light in vacuum: c = 3 x 108 m/s
So they don't need medium in order to travel. They don't need anything to be there at all.
See the image, All of these are "electromagnetic waves".This means they're all made of the same kind of vibrations - but at different frequencies.
So by using these technologies, sound waves (ex: "Anybody is there? We are from earth.") Are converted to Radio Waves and spread over the sky in almost all languages.
Just read my previous post "If Sound travel through vacuum...", and then you can understand this last paragraph.