About Nio:

Founded in Nov 2014
First deliveries June 2018 (3 years & 9 months for first delivery)
Market cap 36B
Share price 21.88 close on March 29th (As I'm writing this it's trading at 23.50~ a 7.5% day increase)
2018 total deliveries 11,348 -> 2019 total deliveries 20,565 (81.22% increase over previous year) -> 2020 total deliveries 43,728 (112.63% increase over previous year) -> 2021 total deliveries 91,429 (109.09% increase over previous year)
Their losses have also been dropping at staggering amounts, from 2020 to 2021 the operation losses have dropped by a whole 41.2% (excluding share based compensation)

So as you can see this company has proven to grow at massive rates, (and btw 3 years ago they escaped bankruptcy) so it's very likely to be incredibly profitable in the future. If they keep their current growth up, they'll reach Tesla's current deliveries within 5 years.

Ok so now I'll be showing why NIO is underpriced through a small comparison with Tesla, if you think Tesla is overpriced. Click away this post since it'll be a pointless read. (I do agree that Tesla is overpriced at it's current profits, however their future profit potential makes up for it.)

Well the comparison is simple really. If NIO has a market cap of 36B at 91K deliveries. And Tesla has a market cap of 1.146T at 936K deliveries u could translate that into NIO deserving a market cap around 1/10th of Tesla's. This would be something around 90-120B. This leads me to believe that NIO is currently selling at 1/3rd-1/4th of it's true value.

Now as to why it's rallying today, is all due to the promising future it shows and more light being shined upon it today. Especially after new came out that Ark Invest's (60B asset value) CEO Cathie Woods sold 146.000 TSLA shares on the 25th of march and, bought 420.057 shares of NIO on the same day. Meaning they dropped some of their Tesla stocks for the booming NIO stock.

One of the main reasons that NIO is so cheap right now is due to distrust in China, however if you don't experience that and have trust in the EV future and companies such as Tesla, Nio is a ridiculously cheap stock right now with an analyst price target around $40-50 and high estimates of $90.

Sources: NIO Inc. Reports Unaudited Fourth Quarter and Full Year 2019 Financial Results | NIO Inc. , NIO Inc. Reports Unaudited Fourth Quarter and Full Year 2020 Financial Results | NIO Inc. , NIO Inc. Reports Unaudited Fourth Quarter and Full Year 2021 Financial Results | NIO Inc. , Tesla Investor Relations , Why Cathie Wood Just Dumped Tesla for This Hot EV Stock | The Motley Fool , NIO - NIO Inc Forecast - CNNMoney.com&text=The%2028%20analysts%20offering%2012,the%20last%20price%20of%2021.88.) , NIO Price Target & Analyst Ratings (NYSE:NIO) | MarketBeat , NIO Stock Forecast, Price Targets and Analysts Predictions - TipRanks