Question

why temperature of a body doesn't go to below -273 degree celcius or 0 kelvin?

Answer 1

Because at -273 or 0k, velocity of molecules of the body becomes zero.

Answer 2

At the physically impossible-to-reach temperature of zero kelvin, or minus 459.67 degrees Fahrenheit (minus 273.15 degrees Celsius), atoms would stop moving. As such, nothing can be colder than absolute zero on the Kelvin scale.