How do scientists know that the universe is expanding?

It is known that the elements captivate and emit specific wavelengths of light, that is, the part of the electromagnetic spectrum. By examining the wavelengths of light discharged by an object in space, one can attain an ample amount of information. The astronomers studied the variation in the position of lines in the spectrum from a star, this can predict the distance of the star, that is, and whether it is moving away or towards us, and how quickly it is moving.  

While studying the radiation discharged by the galaxies or the distant stars, the scientists witnessed the shifting of emission spectra towards the red region of the electromagnetic spectrum, that is, the witnessed wavelengths are longer than anticipated.  

There is something, which is responsible for the wavelength of the radiation to stretch. However, in spite of a real variation in the wavelength, this procedure was something identical to the Doppler Effect.