They don't. Light from distant stars has red shift, similar to Doppler shift.

IF this red shift is interpreted as Doppler shift, then we can calculate corresponding velocity and then calculate position of a star at today date, and we will see that visible Universe is expanding. However, we need to point to source of energy for that expanding (with acceleration!). Nobody able to find that source yet.

IF this red shift is interpreted as something else (e.g. loses due to friction), then visible Universe is still.