Question

How can the scientists tell that the universe is still expanding?

Answer 1

One evidence of universe's expansion would be the 'redshift'. From a vantage point, scientists have been observing that the electromagnetic radiation that galaxies released such as light is becoming 'redder' in wavelength. It means that the wavelength becomes longer and longer. Thus, they came up with a conclusion that galaxies are moving away from each other and so does the universe relatively.