By Rabbi Hershel Schachter
A very old minhag is recorded in Shulchan Aruch that when a young woman gets married the chupah should take place outside under the stars. Hakodosh Boruch Hu told Avraham Avinu, look at the stars, try to count them, it is impossible, so too, will your descendants be so numerous like the stars in the sky. We want to invoke the blessing that Hashem gave to Avraham Avinu and the Jewish people, that the young woman who is getting married now should be blessed with a lot of children.
Some commentaries on the Shulchan Aruch assume that this minhag existed already in the days of the Talmud Yerushalmi. Tosfos quotes the Talmud Yerushalmi as distinguishing between the chupah of a woman getting married for the first time as opposed to an almonah: standing under the canopy only constitutes nissuin when the woman is getting married for the first time but not if she is an almonah.
Rav Chaim Volozhiner argued that this makes no sense whatsoever; why should we distinguish between a first marriage and a second marriage? Some of the contemporaries of Rav Chaim Volozhiner explained that the idea behind the distinction in the Yerushalmi is not so much first marriage vs. second marriage; but rather that a young woman gets married outside, and the chupah which has four poles constructed in the shape of a tzuras ha'pesach (i.e. what we use to make an eruv for Shabbos purposes), only constitutes a separate room when it is out in the open. But in the case of the older almonah, who is beyond the child bearing age, who gets married inside, because we are not interested in invoking the blessing of "ko yihiye zaracho", the four poles of the chupah do not accomplish that that area should be considered a separate room because the room is surrounded all around with walls which enclose the entire area, including the chupah area, and everything is considered one big room. (The whole idea that standing the under chupah accomplishes nissuin is based on the premise that the chosson and kallah who are married already enter together in a separate room for the sake of accomplishing nissuin).
When Hashem told Avraham Avinu to gaze up at the stars and attempt to count them, the Torah tells us that he was inside his tent and Hashem took him outside to look at the stars. Rashi in his commentary on chumash quotes from the midrash that the idea behind taking him outside was to demonstrate that the Jewish people are not part of the natural order of the world. The Torah tells us that Sara Imeinu was an akorah, and the gemorah tells us that Avraham Avinu as an akor. According to the rules of nature they should not have been able to have children. According to the rules of nature, the entire Jewish people should not have existed. The entire history of the Jewish people is l'maaleh min ha'tevah. This is what the gemorah means by the statement "ein mazel l'Yisroel". It does not mean that Jewish people have no luck, rather it means that our history is not subject to sh'litas ha'kochavim u'mazolos and does not follow the normal rules and regulations of history. One might refer to this concept as "austritt". Hashem told Avraham Avinu to go out of his house, representing the idea that the Jewish people don't really blend in with the rest of nature.
The old minhag of having the chupah under the stars for a young woman getting married is not really being fulfilled if they stand inside the building and open up the ceiling. Even though they are standing under the stars, but they have not walked outside. If the chupah takes place inside the building with just the ceiling removed, according to the interpretation that we quoted in the Talmud Yerushalmi, such a chupah would not be valid. To properly observe the minhag, the chosson and kallah should go out of the building and there, outdoors, stand under the stars.
This idea could perhaps explain the mysterious phenomenon of anti-Semitism throughout all the ages. The Jewish people really don't blend in with the rest of nature. We know that there is a natural tendency for the human body to reject foreign matter. Because the Jewish people have been designated by Hashem as something "chutz min ha'tevah", and perhaps that is why Avraham Avinu refers to himself as ger v'toshav a'nochi I'mochem, the Jews are always considered like geirim, strangers, and correctly so, and this perhaps is the cause of the natural tendency for them to be isolated.
 אבן העזר סי' ס"א סעיף א' Also see Ma'asei Avos Siman LaBonim
 תוס' יומא (י"ג:) ד"ה לחדא
 ספר כתר ראש (סי' צ"ה). ועי' בספו עקבי הצאן (עמוד 265-266) .
 עי' יבמות (ס"ד.)
 שבת( קנ"ו.)
 בפ' חיי שרה (בראשית כ"ג-ד')
 See The Meaning of Am Hanivchar; the Source of Anti-Semitism