About

The Invisible Pink Unicorn (IPU) is the goddess of a parody religion used to satirize theistic beliefs, taking the form of a unicorn that is paradoxically both invisible and pink. This makes her a rhetorical illustration used by atheists and other religious skeptics.

(Source: Wikipedia, see link)

Nowadays, the Invisible Pink Unicorn has become a metaphor to depict atheists, for it is some kind of spearhead that is used as an illustration of the problem faced by atheists when asked to disprove the existence of a god by those of a religious persuasion. Due to her invisibility, the existence of the IPU is impossible to disprove, and this is doubly so due to the fact that, according to some, only her believers can see her.
In brief, it's a key-character to point at Religion's paradoxical, as well as sometimes nonsensical, facts.
At the same time, it is a parody of religion similar to The Flying Spaghetti Monster.

Pre-internet era: Evolution of skepticism

The concept of using offbeat metaphors to point at religious somewhat oppressive ideas and to justify the right to be skeptic about their claims isn't new.
In the mid-1950s, Bertrand Russell, British philosopher and mathematician, was one of the first to offer a theory called Russel's teapot as an example to assert that it isn't up to scientists to prove an inexistence.
Here is the complete text, entitled Is There a God?, according to wikipedia:

If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is an intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense. If, however, the existence of such a teapot were affirmed in ancient books, taught as the sacred truth every Sunday, and instilled into the minds of children at school, hesitation to believe in its existence would become a mark of eccentricity and entitle the doubter to the attentions of the psychiatrist in an enlightened age or of the Inquisitor in an earlier time

That idea of a celestial teapot, that would be reused in some contemporary works is believed to be the precursor that would be extrapoled in more straightforward parodies of Religions (including the Invisible Pink Unicorn).

Origin

The earliest known reference to an Invisible Pink Unicorn came from a 1990 Usenet group talk, in a channel called alt.atheism.
In it, someone created a topic about Proof of God's existence (talk archived) to challenge atheist ideas.

While coming to the Christ's ressurection subject, here's the response he got:

Do you assert a resurrection? If so, support it with evidence. If you feel it
is the duty of others to refute the claim, and not of yourself to support it,
then how about refuting the existence of invisible pink unicorns?

(Bold added for emphasis)

Church of the Invisible Pink Unicorn

According to wikipedia and other articles, during the years 1994 – 1995, a group of college students wrote a manifesto, that is now believed to be lost, on the ISCA Telnet-based BBS containing the main roots of what will further be the IPU religion.
From that manifesto only remains the most notable quote that has become popular amongs followers:

"Invisible Pink Unicorns are beings of great spiritual power. We know this because they are capable of being invisible and pink at the same time. Like all religions, the Faith of the Invisible Pink Unicorns is based upon both logic and faith. We have faith that they are pink; we logically know that they are invisible because we can't see them." -- Steve Eley

You can find all the basics concerning IPU's "Bible" in this 2007 article:
"https://h2g2.com/edited_entry/A18592815

This "Religion" got its own logo, that will then be used as a logo for atheism:

Popularity on Internet

This "Religion", since its creation, has gathered many followers.
Starting around 2001, websites dedicated to that myth have been created:

• Invisible Unicorn Web
• The Vegnautic sect of the Invisible Pink Unicorn
• Invisible Pink Unicorn
• The IPU
• The Invisible Pink Unicorn – Peace Be Unto Her
• Monument of the Invisible Pink Unicorn
• Uncyclopedia article

- Urban Dictionary entry from 2004

- 2 Ytmnds in 2006:

• Mr.T and atheists
• IPU

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2011 Easter appearance

On April 25th 2011, an Italian group called Les Liens Invisibles^[1] shot a video during the Easter Vatican Service at the Piazza San Pietro in Roma, in which they made the Invisible Pink Unicorn appear using Layar's Augmenter Reality application for Smartphones :

That event has been relayed by Internet outlets, including Wired^[2] and Further Field.com^[3]. It's also hosted on a dedicated website^[4].

Youtube Videos

External References

^[1] Les Liens Invisibles – The Invisible Pink Unicorn: Art overtakes Faith in imagination

^[2] Wired news – Augmented Reality: Invisible Pink Unicorn at the Easter Vatican Service

^[3] Further Field – The Invisible Pink Unicorn: Art overtakes Faith in imagination

^[4] Invisibile Unicorno Rosa – Invisibile Unicorno Rosa – possano i Suoi Zoccoli non essere mai ferrati (in Italian)