"In the OT, ancient Jewish sources, and the NT, angels have the appearance of human beings and can be mistaken for them (see, e.g., Gen 18:2, l6, 22; 19:1; 2 Macc 10:29–31; 11:8–12). In Judg 13:6 an angel is called “a man of God,” and in Acts 1:10–11 a pair of interpreting angels is referred to as “two men in white robes.” In 2 Macc 3:26, 33 and Josephus, Ant. 5.277, moreover, angels are called neaniai (“young men”), and on icons angels characteristically have a youthful appearance (cf. Allison, Testament of Abraham, 95). Furthermore, our passage is formally similar to the angelophanies in Dan 10:2–14; Matt 1:18–25; Apoc. Abr. 10:1–17; and 2 En. 1:3–10, which include an introductory identification of the recipients, a description of the angel appearing, a reference to the recipients’ fear, a word of consolation from the angel, a word of revelation, and usually a word of command (see Davies and Allison, 2.660–61). One typical component, however, is conspicuously missing in the Markan case: the recipients’ obedient response to the command." (Joel Marcus Mark 8-16, Anchor Yale Bible Commentaries, 2009, Page 1077)
"When they see a young man clad in a white robe, he is sitting on the tomb’s right-hand side (16:5), a position traditionally associated with power, victory, and auspiciousness (cf. Mark 12:36; 14:62; 1 Kgs 2:19; Xenophon, Cyropaedia 2.1.1; b. Hor. 12a; Gornatowski, Rechts; Grundmann, “Dexios,” 3738). Although Mark does not call this youth an angel, that is probably what he understands him to be, since biblical angels have the appearance of human beings (see the NOTE on “a young man” in 16:5). That the angel is called a young man (neaniskon) is significant as well; his youthful appearance suggests the freshness and vigor of the new era that has just dawned (see Peter Chrysologus, Sermon 82 and cf. 2 Macc 3:26, 33). It is unclear, however, whether the women recognize him as a supernatural being; when angels are called “men” or “youths” in the Bible, that is usually because they appear human to the people who encounter them. It is possible, then, that the women’s initial astonishment has to do with finding a living being in the tomb rather than with immediately recognizing him as an angel. But it seems more likely that they do perceive him as an angel, since their reaction of fear, and his reassurance, correspond to a typical pattern in biblical angelophanies (see, e.g., Judg 6:22–23; Dan 8:17; 10:7, 12; Tob 12:16–17; Luke 2:9–10; Rev 1:17). "
(Joel Marcus Mark 8-16, Anchor Yale Bible Commentaries, 2009, Page 1085)
TL;DR: The young man seems like an angel.
Mark says that the women told nobody, but this is a part of Mark's partially/mostly a-historical messianic secret. Adela Y. Collins writes:
"In 16:8*, however, the cause of the fear is clear from the context. It points backward, not forward. In the first part of the verse, it is clear that the trembling and amazement that seized the women were caused by what they had seen and heard in the tomb. The disappearance of Jesus’ body, the presence of an angel, and the announcement that Jesus had risen from the dead are events that go beyond, or even contradict, ordinary expectations and experience. The second part of v. 8*, “and they said nothing to anyone; for they were afraid,” continues the description of the impact that the experience at the tomb had on the women. Their silence is a result of their being struck with awe at the extraordinary events. The tension between the commission given the women by the angel in v. 7* and the silence of the women in v. 8* is due to the depiction of the overwhelming effect of the overall experience on the women. The text does not address the question whether the women eventually gave the disciples and Peter the message. It focuses rather on the numinous and shocking character of the event of Jesus’ resurrection from the dead.
This interpretation is supported by primary texts within and outside of Mark. In the account of the stilling of the storm, the disciples are presented indirectly as having an ordinary fear of the storm when they wake Jesus and say “don’t you care that we are perishing?” (4:38*). When Jesus has put a stop to the storm, however, they experience a different kind of fear (4:41*). They are terrified when they see Jesus, whom they presume to be an ordinary human being, act with divine power. Furthermore, a reaction of amazement or awe is typical of the responses of those present to the mighty deeds of Jesus. This kind of reaction is related to the typical human response to a theophany or epiphany.
A good example of overwhelmed, awestruck flight in response to a divine epiphany occurs in a hymn to Demeter by Callimachus. Demeter, disguised as her priestess, warns a youth to stop cutting down her sacred trees. Rather than desist, he defies her:
But Demeter was unspeakably enraged, and took on her godlike shape again: her steps touched the ground, but her head touched Olympus. When they [the young man and his servants] saw the goddess they started away, half-dead with fear, leaving their bronze implements in the trees (Δαμάτηρ δʼ ἄφατόν τι κοτέσσατο, γείνατο δʼ αὖ θεύς· ἴθματα μὲν χέρσω, κεφαλὰ δέ οἱ ἅψατʼ Ὀλύμπω. οἱ μὲν ἄρʼ ἡμιθνῆτες, ἐπεὶ τὰν πότνιαν εἶδον, ἐξαπίνας ἀπόρουσαν ἐνὶ δρυσὶ χαλκὸν ἀφέντες). (Callimachus Hymn VI [to Demeter] 57–60)
In Greek literature, “Fear is a common reaction to divine epiphany.” In biblical literature, the appearance of an angel is analogous to the Greek divine epiphany"
(Adela Y. Collins, Mark: A Commentary, Hermeneia, 2007, p. 799-800)
A number of scholars posit that the Markan empty tomb signals that an apotheosis ("to deify") took place, and that Jesus was translated straight to heaven, and that Mark has no need for Christophanies (appearances). The vanishing of the corpse signals the transformation to deity and thus Mark ends on its natural climax: Christ has achieved deification.
For example, regarding Augustus’s death, Suetonius says:
There was even an ex-praetor who took oath that he had seen the form of the Emperor, after he had been reduced to ashes, on its way to heaven (Nec defuit vir praetorius, qui se effigiem cremati euntem in caelum vidisse iuraret). (Aug. 100.4)
Regarding Caesar’s deification, Pliny says:
He died in the fifty-sixth year of his age, and was numbered among the gods, not only by a formal decree, but also in the conviction of the common people. For at the first of the games which his heir Augustus gave in honor of his apotheosis, a comet shone for seven successive days, rising about the eleventh hour [about an hour before sunset] and was believed to be the soul of Caesar, who had been taken to heaven; and this is why a star is set upon the crown of his head in his statue . (Jul. 88)
Adela Y. Collins writes:
The author of Mark was probably aware of the idea that some Roman emperors had ascended to heaven and become gods. He may also have known that their deifications were modeled on that of Romulus. (Ibid, 793)
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This seems unlikely though, and this view is not without it's objections. "To deify" implies the being in question wasn't divine before the translation event, and this is not what we see in Mark. John Granger Cook writes:
"The narrative is not an apotheosis... Jesus was already divine according to the text of the Gospel.\1]) ... Jesus is to appear in Galilee before any presumed ascension to heaven, and this is fundamentally different from most translation narratives. No account of a Greco Roman translation I am aware of or any apotheosis of an emperor uses a verb for resurrection."
(John Granger Cook, Empty Tomb, Resurrection, Apotheosis, Mohr Siebeck, 2018, p. 597-598)
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1 D. A. Smith, Revisiting the Empty Tomb. The Early History of Easter, Minneapolis 2010, 77–8 seems unaware that Mark 16:6 renders any speculation that “an empty tomb story need not presuppose resurrection” useless. Since such speculation is unverifiable, there is no need to engage in it. Mark’s story presupposes resurrection and not translation. (Ibid, 597)
(John Granger Cook, Empty Tomb, Resurrection, Apotheosis, Mohr Siebeck, 2018, p. 597-598)
For more on this, check out Miller 2010.