I'm just wondering if Kaguya Ōtsutsuki is an extraterrestrial being came form outher space.
EDIT: Leaving the old answer as it was mostly accurate at the time of answering, but new canon after the manga ended via the movies, non manga sources, and Boruto series have changed it.
Kaguya was in fact an alien. As revealed in the Boruto movie and series, Kaguya was from another planet/dimension. The reason she came to Naruto's planet is believed to be because it had a Chakra fruit and Shinju tree. What is clear is that she knew of other Ōtsutsuki and feared them, building the White Zetsu army to help defeat them in battle when they came, but its not clear if she ran away or was sent there and effectively betrayed her clan due to corruption caused by all the power she obtained, especially considering Momoshiki's dialog before he departed for earth.
As time passed, however, Kaguya began to lose trust in humanity as the power she now held ultimately corrupted her, which changed her public image from that of a benevolent goddess into that of a Demon (鬼, Oni). This eventually led her to madness, committing acts such as turning victims of the Infinite Tsukuyomi into an army of White Zetsu, in preparation for Ōtsutsuki descendants from another world coming to Earth to steal her chakra for themselves.
Momoshiki was approached by Kinshiki, who had observed substantial tears in Kaguya's world, with its space curvature exceeding 16. After Kinshiki noted that she violated their clan's laws by failing to report the abnormal space curvature, Momoshiki wondered whether she had betrayed the clan, or had somehow been overthrown and destroyed by the "inferior lifeforms" inhabiting her world.
The rest of the old answer (again, apart from the fact that she is in fact an alien) is correct, at least from the perspective of the original manga, or more specifically Hagoromo.
Kaguya was a Human Who ate the Chakra fruit of the Shinju tree
"A millennium ago, Kaguya, longing for peace, believed that it was necessary for her to attain god-like powers in order to put an end to all of humanity's conflicts. For that reason, she defied the taboo of consuming the fruit of the Shinju. Using her new-found power, Kaguya single-handedly ended the wars, and for her deed, she was worshiped as the Rabbit Goddess"
No events before this are clearly described beyond wars and violence, but it is implied that Humans before were fairly normal, just like all the normal non ninja citizens of the Villages. Her powers did include Dimensional shifting, as shown in the manga, but the only known case of leaving earth is also attributed to Kaguya, when her Sons defeated her and Sealed her body within a celestial body which became the moon.
In a fit of envy that her own sons had inherited her chakra, Kaguya combined with the Shinju itself and turned against them in the form of the monstrous Ten-Tails. Their intense battle ended with Hagoromo and Hamura separating and absorbing their mother's powerful chakra while sealing her body to form the core of a celestial body that would become known as the moon.
So if anything, She is just an unwilling Astronaut.
EDIT: NOTE the anime arc involving Kaguya and Hagoromo's past is not in the manga, and mostly filler. We never actually saw Kaguya eat the fruit or know who Hagoromo and Hamura's Father is in the Manga, or if Hagoromo ever had the sharingan in any form. The anime even changed Indra's eyes to look like Sasuke's stars, but in the manga, they were swirls, though the wiki does not have any pictures of Indra in the manga, the Mangekyou Sharingan page does describe this in at the bottom with a picture of his pattern and a Note that the anime changed it.
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Kaguya belongs to the Otsutsuki clan. And here's what Narutopedia says about the clan in the very first sentence:
The Ōtsutsuki clan (大筒木一族, Ōtsutsuki Ichizoku) is a family of extra-terrestrials.
Well there you go. Not too hard to just search the answer for yourself online, eh?