How could Domitian subdue Galba if Galba was killed by Otho amd how could Domitian subdue Otho if Otho killed himself? I’m confused.
I am not sure where you are getting your information. However, I assume you have read material critical of the prophecy in Daniel 7. This is the prophecy which predicted that the eleventh emperor of Rome (i.e. Domitian) would overcome three horns (i.e. three former emperors of Rome). Daniel 7 is a vision of Domitian, his incredible pride, his changing of the times (the names of the months in Rome), his attack on the saints (which resulted, for example, in John being sent as an exile to Patmos), his changing of the laws (he overturned the entire Roman legal system).
The most important primary source for Galba, Otho and Vitellius is Suetonius and his work on the early emperors of Rome. Suetonius confirms nearly all of Daniel’s prophecies as listed above. Suetonius does not provide every possible detail of the tumultuous times immediately after the death of Nero. Nero was the last of the Claudo/Julian dynasty, leaving no clear heir to the Roman Empire. On his death, the most obvious candidate was Vespasian. He was the preeminant military leader of the empire. However, Vespasian was at the opposite end of the empire from Rome, besieging Jerusalem (and fulfilling the prophecy in Daniel 9 and Matthew 24). After making the situation at Jerusalem secure, and leaving his son Titus (future emperor of Rome after Vespasian and before Domitian) to finish the capture and destruction of Jerusalem, Vespasian rushed to Rome to take what he considered to be rightfully his.
In the meantime, Galba, Otho and Vitellius successively claimed the throne. Their carreers are described in some detail by Suetonius. None of the three was able to win over more than a minority of the legions. Domitian, the other son of Vespasian, remained in Rome, working behind the scenes to prepare the way for his father to take the place of emperor of Rome. It is true that Suetonius only reports Domitian in the field of battle against the last of the three-Vitellius. He does not tell us every detail of the battles and intrigues going on behind the scenes. We can be assured that Domitian was working to undermine all three of these pseudo-emperors. Although we do not have a record of him actually engaged in battles leading troops against Galba and Otho, it is very likely that he did this. In any case, he was certainly a leading figure in undermining all three of the pretenders to the throne (pretenders, from his perspective, that is).
The fact that Otho was the general in charge when Galba was defeated and that Otho killed himself after being defeated does not invalidate the evidence that Domitian was involved in the overthrow of all three. As they say, "Absence of evidene is not evidence of absence." Given the uncanny accuracy of Daniel’s prophecy–made over six hundred years before the events–it is not unreasonable at all to "give the benefit of the doubt" to Daniel’s accuracy, given that the facts prove Domitian overthrew Vitellius and suggest that he was also involved in the overthrow of the other two, even if we have no direct evidence of his taking the military field against the other two.