What are the most obscure kanji?

Question

To my understanding, there are standardised kanji that have either:

• No reliable information on how they are to be pronounced OR
• No reliable information on what they mean OR
• All of the above.

Do these kanji exist, and if so: What are the current hypotheses on what they mean/how they are pronounced?

Answer 1

We call them:

• No reliable information on how they are to be pronounced → 音【おん】（未詳【みしょう】／不明【ふめい】）字【じ】
• No reliable information on what they mean → 義【ぎ】（未詳／不明）字
• All of the above → 音義（未詳／不明）字

幽霊文字 in @naruto's answer is usually considered as a subset of them (technically, meaning/pronunciation of some 幽霊文字 are easily inferrable so they're not true subset).

Some Chinese Characters have unknown meaning/pronunciation even they have reliable source, because it's clearly written in dictionaries that "音未詳"/"義未詳"/"音義未詳". When those massive dictionaries were compiled, they tried to collect characters from all available documents. Some characters only have too sporadic (or single) specimens to reasonably guess what they are. Some others are from previous dictionary with whose description about meaning/pronunciation are lost or unrecorded. The Unicode standard contains a numerous number of those characters because it has unconditionally incorporated all entries in the authoritative dictionaries.

While they have possibility to be identified through investigation, there was no research encompassing the entire encoded characters as far as I know, thus many of them are just left to be determined.

• 「JIS X 0208」における音義未詳字に対する原典による同定
• JIS音義未詳字の辞典
• 諸橋轍次編『大漢和辞典』の＜音義未詳字＞
• 《大漢和辞典》第一巻中の難字1 to 大漢和辞典の未詳字23

Answer 2

Yes.

幽霊文字
https://ja.wikipedia.org/wiki/%E5%B9%BD%E9%9C%8A%E6%96%87%E5%AD%97

幽霊文字（ゆうれいもじ）とは、JIS基本漢字に含まれる、典拠不明の文字の総称。

JIS基本漢字 (aka JIS第1・第2水準漢字) is a set of Japanese characters (including approx. 6350 kanji) established in 1978. It has been widely supported by personal computers since then, but this standard contains several kanji which have no reliable origin.

For example, 彁 is a kanji which probably did not exist before the JIS基本漢字 set was established. No one knows its meaning. It has the reading (カ/セイ), but it's a fabricated one for convenience sake.

戸籍統一文字 contains over 50,000 "kanji", many of which are not listed in any dictionary. They have character codes and actually in use in government offices, but I don't know whether you can call them "standardized".

And you can find tons of insane kanji which are not standardized here: 漢字部屋