Question

why was it necessary to define a new unit to measure distance in the universe what is this unit

Answer 1

The distances between objects in space are so vast that a new unit of measurement of distances was needed. This new unit was 'light year'.
One light year is defined as distance covered by light in one year and 1 light year = 9.46 x1015 m

Answer 2

The distances between heavenly bodies in space are very, very large. Measuring these distances in kilometres

would mean dealing with very large numbers. Astronomers, therefore, use special units to measure distances in

space. Instead of a kilometre, they use a unit called light year. A light year is the distance that light travels in one year.