"Domitia" wasn't an emperor; the "Empire" didn't exist back in 128 BC. This coin was issued during the time of the Republic. The portrait is that of Roma, personification of Rome, and not intended to resemble a specific person.
During this period, the Mint was under the control of one of the senior Consuls, or elected officials, and the coins are catalogued according to which noble family the mint official at the time belonged to. Thus, "Domitia" is the gens, or family name, for a fellow named "Domitius".
They're not too sure exactly who this coin was issued by; apparently a "Domitius Ahenobarbus" and a "Domitius Calvinus" are both known to history. Whoever it was, his initials, "CN DOM", appear below the chariot.
The coi is listed in Sear as number 137.